diff --git a/packages/react-dom/src/__tests__/ReactLegacyUpdates-test.js b/packages/react-dom/src/__tests__/ReactLegacyUpdates-test.js
new file mode 100644
index 0000000000000..d1e54dbb055ca
--- /dev/null
+++ b/packages/react-dom/src/__tests__/ReactLegacyUpdates-test.js
@@ -0,0 +1,1591 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+let React;
+let ReactDOM;
+let ReactTestUtils;
+let act;
+let Scheduler;
+let assertLog;
+
+// Copy of ReactUpdates using ReactDOM.render and ReactDOM.unstable_batchedUpdates.
+// Can be deleted when we remove both.
+describe('ReactLegacyUpdates', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactDOM = require('react-dom');
+    ReactTestUtils = require('react-dom/test-utils');
+    act = require('internal-test-utils').act;
+    Scheduler = require('scheduler');
+
+    const InternalTestUtils = require('internal-test-utils');
+    assertLog = InternalTestUtils.assertLog;
+  });
+
+  it('should batch state when updating state twice', () => {
+    let updateCount = 0;
+
+    class Component extends React.Component {
+      state = {x: 0};
+
+      componentDidUpdate() {
+        updateCount++;
+      }
+
+      render() {
+        return <div>{this.state.x}</div>;
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
+    expect(instance.state.x).toBe(0);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.setState({x: 1});
+      instance.setState({x: 2});
+      expect(instance.state.x).toBe(0);
+      expect(updateCount).toBe(0);
+    });
+
+    expect(instance.state.x).toBe(2);
+    expect(updateCount).toBe(1);
+  });
+
+  it('should batch state when updating two different state keys', () => {
+    let updateCount = 0;
+
+    class Component extends React.Component {
+      state = {x: 0, y: 0};
+
+      componentDidUpdate() {
+        updateCount++;
+      }
+
+      render() {
+        return <div>{`(${this.state.x}, ${this.state.y})`}</div>;
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
+    expect(instance.state.x).toBe(0);
+    expect(instance.state.y).toBe(0);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.setState({x: 1});
+      instance.setState({y: 2});
+      expect(instance.state.x).toBe(0);
+      expect(instance.state.y).toBe(0);
+      expect(updateCount).toBe(0);
+    });
+
+    expect(instance.state.x).toBe(1);
+    expect(instance.state.y).toBe(2);
+    expect(updateCount).toBe(1);
+  });
+
+  it('should batch state and props together', () => {
+    let updateCount = 0;
+
+    class Component extends React.Component {
+      state = {y: 0};
+
+      componentDidUpdate() {
+        updateCount++;
+      }
+
+      render() {
+        return <div>{`(${this.props.x}, ${this.state.y})`}</div>;
+      }
+    }
+
+    const container = document.createElement('div');
+    const instance = ReactDOM.render(<Component x={0} />, container);
+    expect(instance.props.x).toBe(0);
+    expect(instance.state.y).toBe(0);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      ReactDOM.render(<Component x={1} />, container);
+      instance.setState({y: 2});
+      expect(instance.props.x).toBe(0);
+      expect(instance.state.y).toBe(0);
+      expect(updateCount).toBe(0);
+    });
+
+    expect(instance.props.x).toBe(1);
+    expect(instance.state.y).toBe(2);
+    expect(updateCount).toBe(1);
+  });
+
+  it('should batch parent/child state updates together', () => {
+    let parentUpdateCount = 0;
+
+    class Parent extends React.Component {
+      state = {x: 0};
+      childRef = React.createRef();
+
+      componentDidUpdate() {
+        parentUpdateCount++;
+      }
+
+      render() {
+        return (
+          <div>
+            <Child ref={this.childRef} x={this.state.x} />
+          </div>
+        );
+      }
+    }
+
+    let childUpdateCount = 0;
+
+    class Child extends React.Component {
+      state = {y: 0};
+
+      componentDidUpdate() {
+        childUpdateCount++;
+      }
+
+      render() {
+        return <div>{this.props.x + this.state.y}</div>;
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Parent />);
+    const child = instance.childRef.current;
+    expect(instance.state.x).toBe(0);
+    expect(child.state.y).toBe(0);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.setState({x: 1});
+      child.setState({y: 2});
+      expect(instance.state.x).toBe(0);
+      expect(child.state.y).toBe(0);
+      expect(parentUpdateCount).toBe(0);
+      expect(childUpdateCount).toBe(0);
+    });
+
+    expect(instance.state.x).toBe(1);
+    expect(child.state.y).toBe(2);
+    expect(parentUpdateCount).toBe(1);
+    expect(childUpdateCount).toBe(1);
+  });
+
+  it('should batch child/parent state updates together', () => {
+    let parentUpdateCount = 0;
+
+    class Parent extends React.Component {
+      state = {x: 0};
+      childRef = React.createRef();
+
+      componentDidUpdate() {
+        parentUpdateCount++;
+      }
+
+      render() {
+        return (
+          <div>
+            <Child ref={this.childRef} x={this.state.x} />
+          </div>
+        );
+      }
+    }
+
+    let childUpdateCount = 0;
+
+    class Child extends React.Component {
+      state = {y: 0};
+
+      componentDidUpdate() {
+        childUpdateCount++;
+      }
+
+      render() {
+        return <div>{this.props.x + this.state.y}</div>;
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Parent />);
+    const child = instance.childRef.current;
+    expect(instance.state.x).toBe(0);
+    expect(child.state.y).toBe(0);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      child.setState({y: 2});
+      instance.setState({x: 1});
+      expect(instance.state.x).toBe(0);
+      expect(child.state.y).toBe(0);
+      expect(parentUpdateCount).toBe(0);
+      expect(childUpdateCount).toBe(0);
+    });
+
+    expect(instance.state.x).toBe(1);
+    expect(child.state.y).toBe(2);
+    expect(parentUpdateCount).toBe(1);
+
+    // Batching reduces the number of updates here to 1.
+    expect(childUpdateCount).toBe(1);
+  });
+
+  it('should support chained state updates', () => {
+    let updateCount = 0;
+
+    class Component extends React.Component {
+      state = {x: 0};
+
+      componentDidUpdate() {
+        updateCount++;
+      }
+
+      render() {
+        return <div>{this.state.x}</div>;
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
+    expect(instance.state.x).toBe(0);
+
+    let innerCallbackRun = false;
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.setState({x: 1}, function () {
+        instance.setState({x: 2}, function () {
+          expect(this).toBe(instance);
+          innerCallbackRun = true;
+          expect(instance.state.x).toBe(2);
+          expect(updateCount).toBe(2);
+        });
+        expect(instance.state.x).toBe(1);
+        expect(updateCount).toBe(1);
+      });
+      expect(instance.state.x).toBe(0);
+      expect(updateCount).toBe(0);
+    });
+
+    expect(innerCallbackRun).toBeTruthy();
+    expect(instance.state.x).toBe(2);
+    expect(updateCount).toBe(2);
+  });
+
+  it('should batch forceUpdate together', () => {
+    let shouldUpdateCount = 0;
+    let updateCount = 0;
+
+    class Component extends React.Component {
+      state = {x: 0};
+
+      shouldComponentUpdate() {
+        shouldUpdateCount++;
+      }
+
+      componentDidUpdate() {
+        updateCount++;
+      }
+
+      render() {
+        return <div>{this.state.x}</div>;
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Component />);
+    expect(instance.state.x).toBe(0);
+
+    let callbacksRun = 0;
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.setState({x: 1}, function () {
+        callbacksRun++;
+      });
+      instance.forceUpdate(function () {
+        callbacksRun++;
+      });
+      expect(instance.state.x).toBe(0);
+      expect(updateCount).toBe(0);
+    });
+
+    expect(callbacksRun).toBe(2);
+    // shouldComponentUpdate shouldn't be called since we're forcing
+    expect(shouldUpdateCount).toBe(0);
+    expect(instance.state.x).toBe(1);
+    expect(updateCount).toBe(1);
+  });
+
+  it('should update children even if parent blocks updates', () => {
+    let parentRenderCount = 0;
+    let childRenderCount = 0;
+
+    class Parent extends React.Component {
+      childRef = React.createRef();
+
+      shouldComponentUpdate() {
+        return false;
+      }
+
+      render() {
+        parentRenderCount++;
+        return <Child ref={this.childRef} />;
+      }
+    }
+
+    class Child extends React.Component {
+      render() {
+        childRenderCount++;
+        return <div />;
+      }
+    }
+
+    expect(parentRenderCount).toBe(0);
+    expect(childRenderCount).toBe(0);
+
+    let instance = <Parent />;
+    instance = ReactTestUtils.renderIntoDocument(instance);
+
+    expect(parentRenderCount).toBe(1);
+    expect(childRenderCount).toBe(1);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.setState({x: 1});
+    });
+
+    expect(parentRenderCount).toBe(1);
+    expect(childRenderCount).toBe(1);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      instance.childRef.current.setState({x: 1});
+    });
+
+    expect(parentRenderCount).toBe(1);
+    expect(childRenderCount).toBe(2);
+  });
+
+  it('should not reconcile children passed via props', () => {
+    let numMiddleRenders = 0;
+    let numBottomRenders = 0;
+
+    class Top extends React.Component {
+      render() {
+        return (
+          <Middle>
+            <Bottom />
+          </Middle>
+        );
+      }
+    }
+
+    class Middle extends React.Component {
+      componentDidMount() {
+        this.forceUpdate();
+      }
+
+      render() {
+        numMiddleRenders++;
+        return React.Children.only(this.props.children);
+      }
+    }
+
+    class Bottom extends React.Component {
+      render() {
+        numBottomRenders++;
+        return null;
+      }
+    }
+
+    ReactTestUtils.renderIntoDocument(<Top />);
+    expect(numMiddleRenders).toBe(2);
+    expect(numBottomRenders).toBe(1);
+  });
+
+  it('should flow updates correctly', () => {
+    let willUpdates = [];
+    let didUpdates = [];
+
+    const UpdateLoggingMixin = {
+      UNSAFE_componentWillUpdate: function () {
+        willUpdates.push(this.constructor.displayName);
+      },
+      componentDidUpdate: function () {
+        didUpdates.push(this.constructor.displayName);
+      },
+    };
+
+    class Box extends React.Component {
+      boxDivRef = React.createRef();
+
+      render() {
+        return <div ref={this.boxDivRef}>{this.props.children}</div>;
+      }
+    }
+    Object.assign(Box.prototype, UpdateLoggingMixin);
+
+    class Child extends React.Component {
+      spanRef = React.createRef();
+
+      render() {
+        return <span ref={this.spanRef}>child</span>;
+      }
+    }
+    Object.assign(Child.prototype, UpdateLoggingMixin);
+
+    class Switcher extends React.Component {
+      state = {tabKey: 'hello'};
+      boxRef = React.createRef();
+      switcherDivRef = React.createRef();
+      render() {
+        const child = this.props.children;
+
+        return (
+          <Box ref={this.boxRef}>
+            <div
+              ref={this.switcherDivRef}
+              style={{
+                display: this.state.tabKey === child.key ? '' : 'none',
+              }}>
+              {child}
+            </div>
+          </Box>
+        );
+      }
+    }
+    Object.assign(Switcher.prototype, UpdateLoggingMixin);
+
+    class App extends React.Component {
+      switcherRef = React.createRef();
+      childRef = React.createRef();
+
+      render() {
+        return (
+          <Switcher ref={this.switcherRef}>
+            <Child key="hello" ref={this.childRef} />
+          </Switcher>
+        );
+      }
+    }
+    Object.assign(App.prototype, UpdateLoggingMixin);
+
+    let root = <App />;
+    root = ReactTestUtils.renderIntoDocument(root);
+
+    function expectUpdates(desiredWillUpdates, desiredDidUpdates) {
+      let i;
+      for (i = 0; i < desiredWillUpdates; i++) {
+        expect(willUpdates).toContain(desiredWillUpdates[i]);
+      }
+      for (i = 0; i < desiredDidUpdates; i++) {
+        expect(didUpdates).toContain(desiredDidUpdates[i]);
+      }
+      willUpdates = [];
+      didUpdates = [];
+    }
+
+    function triggerUpdate(c) {
+      c.setState({x: 1});
+    }
+
+    function testUpdates(components, desiredWillUpdates, desiredDidUpdates) {
+      let i;
+
+      ReactDOM.unstable_batchedUpdates(function () {
+        for (i = 0; i < components.length; i++) {
+          triggerUpdate(components[i]);
+        }
+      });
+
+      expectUpdates(desiredWillUpdates, desiredDidUpdates);
+
+      // Try them in reverse order
+
+      ReactDOM.unstable_batchedUpdates(function () {
+        for (i = components.length - 1; i >= 0; i--) {
+          triggerUpdate(components[i]);
+        }
+      });
+
+      expectUpdates(desiredWillUpdates, desiredDidUpdates);
+    }
+    testUpdates(
+      [root.switcherRef.current.boxRef.current, root.switcherRef.current],
+      // Owner-child relationships have inverse will and did
+      ['Switcher', 'Box'],
+      ['Box', 'Switcher'],
+    );
+
+    testUpdates(
+      [root.childRef.current, root.switcherRef.current.boxRef.current],
+      // Not owner-child so reconcile independently
+      ['Box', 'Child'],
+      ['Box', 'Child'],
+    );
+
+    testUpdates(
+      [root.childRef.current, root.switcherRef.current],
+      // Switcher owns Box and Child, Box does not own Child
+      ['Switcher', 'Box', 'Child'],
+      ['Box', 'Switcher', 'Child'],
+    );
+  });
+
+  it('should queue mount-ready handlers across different roots', () => {
+    // We'll define two components A and B, then update both of them. When A's
+    // componentDidUpdate handlers is called, B's DOM should already have been
+    // updated.
+
+    const bContainer = document.createElement('div');
+
+    let b;
+
+    let aUpdated = false;
+
+    class A extends React.Component {
+      state = {x: 0};
+
+      componentDidUpdate() {
+        expect(ReactDOM.findDOMNode(b).textContent).toBe('B1');
+        aUpdated = true;
+      }
+
+      render() {
+        let portal = null;
+        // If we're using Fiber, we use Portals instead to achieve this.
+        portal = ReactDOM.createPortal(<B ref={n => (b = n)} />, bContainer);
+        return (
+          <div>
+            A{this.state.x}
+            {portal}
+          </div>
+        );
+      }
+    }
+
+    class B extends React.Component {
+      state = {x: 0};
+
+      render() {
+        return <div>B{this.state.x}</div>;
+      }
+    }
+
+    const a = ReactTestUtils.renderIntoDocument(<A />);
+    ReactDOM.unstable_batchedUpdates(function () {
+      a.setState({x: 1});
+      b.setState({x: 1});
+    });
+
+    expect(aUpdated).toBe(true);
+  });
+
+  it('should flush updates in the correct order', () => {
+    const updates = [];
+
+    class Outer extends React.Component {
+      state = {x: 0};
+      innerRef = React.createRef();
+
+      render() {
+        updates.push('Outer-render-' + this.state.x);
+        return (
+          <div>
+            <Inner x={this.state.x} ref={this.innerRef} />
+          </div>
+        );
+      }
+
+      componentDidUpdate() {
+        const x = this.state.x;
+        updates.push('Outer-didUpdate-' + x);
+        updates.push('Inner-setState-' + x);
+        this.innerRef.current.setState({x: x}, function () {
+          updates.push('Inner-callback-' + x);
+        });
+      }
+    }
+
+    class Inner extends React.Component {
+      state = {x: 0};
+
+      render() {
+        updates.push('Inner-render-' + this.props.x + '-' + this.state.x);
+        return <div />;
+      }
+
+      componentDidUpdate() {
+        updates.push('Inner-didUpdate-' + this.props.x + '-' + this.state.x);
+      }
+    }
+
+    const instance = ReactTestUtils.renderIntoDocument(<Outer />);
+
+    updates.push('Outer-setState-1');
+    instance.setState({x: 1}, function () {
+      updates.push('Outer-callback-1');
+      updates.push('Outer-setState-2');
+      instance.setState({x: 2}, function () {
+        updates.push('Outer-callback-2');
+      });
+    });
+
+    /* eslint-disable indent */
+    expect(updates).toEqual([
+      'Outer-render-0',
+      'Inner-render-0-0',
+
+      'Outer-setState-1',
+      'Outer-render-1',
+      'Inner-render-1-0',
+      'Inner-didUpdate-1-0',
+      'Outer-didUpdate-1',
+      // Happens in a batch, so don't re-render yet
+      'Inner-setState-1',
+      'Outer-callback-1',
+
+      // Happens in a batch
+      'Outer-setState-2',
+
+      // Flush batched updates all at once
+      'Outer-render-2',
+      'Inner-render-2-1',
+      'Inner-didUpdate-2-1',
+      'Inner-callback-1',
+      'Outer-didUpdate-2',
+      'Inner-setState-2',
+      'Outer-callback-2',
+      'Inner-render-2-2',
+      'Inner-didUpdate-2-2',
+      'Inner-callback-2',
+    ]);
+    /* eslint-enable indent */
+  });
+
+  it('should flush updates in the correct order across roots', () => {
+    const instances = [];
+    const updates = [];
+
+    class MockComponent extends React.Component {
+      render() {
+        updates.push(this.props.depth);
+        return <div />;
+      }
+
+      componentDidMount() {
+        instances.push(this);
+        if (this.props.depth < this.props.count) {
+          ReactDOM.render(
+            <MockComponent
+              depth={this.props.depth + 1}
+              count={this.props.count}
+            />,
+            ReactDOM.findDOMNode(this),
+          );
+        }
+      }
+    }
+
+    ReactTestUtils.renderIntoDocument(<MockComponent depth={0} count={2} />);
+
+    expect(updates).toEqual([0, 1, 2]);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      // Simulate update on each component from top to bottom.
+      instances.forEach(function (instance) {
+        instance.forceUpdate();
+      });
+    });
+
+    expect(updates).toEqual([0, 1, 2, 0, 1, 2]);
+  });
+
+  it('should queue nested updates', () => {
+    // See https://github.com/facebook/react/issues/1147
+
+    class X extends React.Component {
+      state = {s: 0};
+
+      render() {
+        if (this.state.s === 0) {
+          return (
+            <div>
+              <span>0</span>
+            </div>
+          );
+        } else {
+          return <div>1</div>;
+        }
+      }
+
+      go = () => {
+        this.setState({s: 1});
+        this.setState({s: 0});
+        this.setState({s: 1});
+      };
+    }
+
+    class Y extends React.Component {
+      render() {
+        return (
+          <div>
+            <Z />
+          </div>
+        );
+      }
+    }
+
+    class Z extends React.Component {
+      render() {
+        return <div />;
+      }
+
+      UNSAFE_componentWillUpdate() {
+        x.go();
+      }
+    }
+
+    const x = ReactTestUtils.renderIntoDocument(<X />);
+    const y = ReactTestUtils.renderIntoDocument(<Y />);
+    expect(ReactDOM.findDOMNode(x).textContent).toBe('0');
+
+    y.forceUpdate();
+    expect(ReactDOM.findDOMNode(x).textContent).toBe('1');
+  });
+
+  it('should queue updates from during mount', () => {
+    // See https://github.com/facebook/react/issues/1353
+    let a;
+
+    class A extends React.Component {
+      state = {x: 0};
+
+      UNSAFE_componentWillMount() {
+        a = this;
+      }
+
+      render() {
+        return <div>A{this.state.x}</div>;
+      }
+    }
+
+    class B extends React.Component {
+      UNSAFE_componentWillMount() {
+        a.setState({x: 1});
+      }
+
+      render() {
+        return <div />;
+      }
+    }
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      ReactTestUtils.renderIntoDocument(
+        <div>
+          <A />
+          <B />
+        </div>,
+      );
+    });
+
+    expect(a.state.x).toBe(1);
+    expect(ReactDOM.findDOMNode(a).textContent).toBe('A1');
+  });
+
+  it('calls componentWillReceiveProps setState callback properly', () => {
+    let callbackCount = 0;
+
+    class A extends React.Component {
+      state = {x: this.props.x};
+
+      UNSAFE_componentWillReceiveProps(nextProps) {
+        const newX = nextProps.x;
+        this.setState({x: newX}, function () {
+          // State should have updated by the time this callback gets called
+          expect(this.state.x).toBe(newX);
+          callbackCount++;
+        });
+      }
+
+      render() {
+        return <div>{this.state.x}</div>;
+      }
+    }
+
+    const container = document.createElement('div');
+    ReactDOM.render(<A x={1} />, container);
+    ReactDOM.render(<A x={2} />, container);
+    expect(callbackCount).toBe(1);
+  });
+
+  it('does not call render after a component as been deleted', () => {
+    let renderCount = 0;
+    let componentB = null;
+
+    class B extends React.Component {
+      state = {updates: 0};
+
+      componentDidMount() {
+        componentB = this;
+      }
+
+      render() {
+        renderCount++;
+        return <div />;
+      }
+    }
+
+    class A extends React.Component {
+      state = {showB: true};
+
+      render() {
+        return this.state.showB ? <B /> : <div />;
+      }
+    }
+
+    const component = ReactTestUtils.renderIntoDocument(<A />);
+
+    ReactDOM.unstable_batchedUpdates(function () {
+      // B will have scheduled an update but the batching should ensure that its
+      // update never fires.
+      componentB.setState({updates: 1});
+      component.setState({showB: false});
+    });
+
+    expect(renderCount).toBe(1);
+  });
+
+  it('throws in setState if the update callback is not a function', () => {
+    function Foo() {
+      this.a = 1;
+      this.b = 2;
+    }
+
+    class A extends React.Component {
+      state = {};
+
+      render() {
+        return <div />;
+      }
+    }
+
+    let component = ReactTestUtils.renderIntoDocument(<A />);
+
+    expect(() => {
+      expect(() => component.setState({}, 'no')).toErrorDev(
+        'setState(...): Expected the last optional `callback` argument to be ' +
+          'a function. Instead received: no.',
+      );
+    }).toThrowError(
+      'Invalid argument passed as callback. Expected a function. Instead ' +
+        'received: no',
+    );
+    component = ReactTestUtils.renderIntoDocument(<A />);
+    expect(() => {
+      expect(() => component.setState({}, {foo: 'bar'})).toErrorDev(
+        'setState(...): Expected the last optional `callback` argument to be ' +
+          'a function. Instead received: [object Object].',
+      );
+    }).toThrowError(
+      'Invalid argument passed as callback. Expected a function. Instead ' +
+        'received: [object Object]',
+    );
+    // Make sure the warning is deduplicated and doesn't fire again
+    component = ReactTestUtils.renderIntoDocument(<A />);
+    expect(() => component.setState({}, new Foo())).toThrowError(
+      'Invalid argument passed as callback. Expected a function. Instead ' +
+        'received: [object Object]',
+    );
+  });
+
+  it('throws in forceUpdate if the update callback is not a function', () => {
+    function Foo() {
+      this.a = 1;
+      this.b = 2;
+    }
+
+    class A extends React.Component {
+      state = {};
+
+      render() {
+        return <div />;
+      }
+    }
+
+    let component = ReactTestUtils.renderIntoDocument(<A />);
+
+    expect(() => {
+      expect(() => component.forceUpdate('no')).toErrorDev(
+        'forceUpdate(...): Expected the last optional `callback` argument to be ' +
+          'a function. Instead received: no.',
+      );
+    }).toThrowError(
+      'Invalid argument passed as callback. Expected a function. Instead ' +
+        'received: no',
+    );
+    component = ReactTestUtils.renderIntoDocument(<A />);
+    expect(() => {
+      expect(() => component.forceUpdate({foo: 'bar'})).toErrorDev(
+        'forceUpdate(...): Expected the last optional `callback` argument to be ' +
+          'a function. Instead received: [object Object].',
+      );
+    }).toThrowError(
+      'Invalid argument passed as callback. Expected a function. Instead ' +
+        'received: [object Object]',
+    );
+    // Make sure the warning is deduplicated and doesn't fire again
+    component = ReactTestUtils.renderIntoDocument(<A />);
+    expect(() => component.forceUpdate(new Foo())).toThrowError(
+      'Invalid argument passed as callback. Expected a function. Instead ' +
+        'received: [object Object]',
+    );
+  });
+
+  it('does not update one component twice in a batch (#2410)', () => {
+    class Parent extends React.Component {
+      childRef = React.createRef();
+
+      getChild = () => {
+        return this.childRef.current;
+      };
+
+      render() {
+        return <Child ref={this.childRef} />;
+      }
+    }
+
+    let renderCount = 0;
+    let postRenderCount = 0;
+    let once = false;
+
+    class Child extends React.Component {
+      state = {updated: false};
+
+      UNSAFE_componentWillUpdate() {
+        if (!once) {
+          once = true;
+          this.setState({updated: true});
+        }
+      }
+
+      componentDidMount() {
+        expect(renderCount).toBe(postRenderCount + 1);
+        postRenderCount++;
+      }
+
+      componentDidUpdate() {
+        expect(renderCount).toBe(postRenderCount + 1);
+        postRenderCount++;
+      }
+
+      render() {
+        expect(renderCount).toBe(postRenderCount);
+        renderCount++;
+        return <div />;
+      }
+    }
+
+    const parent = ReactTestUtils.renderIntoDocument(<Parent />);
+    const child = parent.getChild();
+    ReactDOM.unstable_batchedUpdates(function () {
+      parent.forceUpdate();
+      child.forceUpdate();
+    });
+  });
+
+  it('does not update one component twice in a batch (#6371)', () => {
+    let callbacks = [];
+    function emitChange() {
+      callbacks.forEach(c => c());
+    }
+
+    class App extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = {showChild: true};
+      }
+      componentDidMount() {
+        this.setState({showChild: false});
+      }
+      render() {
+        return (
+          <div>
+            <ForceUpdatesOnChange />
+            {this.state.showChild && <EmitsChangeOnUnmount />}
+          </div>
+        );
+      }
+    }
+
+    class EmitsChangeOnUnmount extends React.Component {
+      componentWillUnmount() {
+        emitChange();
+      }
+      render() {
+        return null;
+      }
+    }
+
+    class ForceUpdatesOnChange extends React.Component {
+      componentDidMount() {
+        this.onChange = () => this.forceUpdate();
+        this.onChange();
+        callbacks.push(this.onChange);
+      }
+      componentWillUnmount() {
+        callbacks = callbacks.filter(c => c !== this.onChange);
+      }
+      render() {
+        return <div key={Math.random()} onClick={function () {}} />;
+      }
+    }
+
+    ReactDOM.render(<App />, document.createElement('div'));
+  });
+
+  it('unstable_batchedUpdates should return value from a callback', () => {
+    const result = ReactDOM.unstable_batchedUpdates(function () {
+      return 42;
+    });
+    expect(result).toEqual(42);
+  });
+
+  it('unmounts and remounts a root in the same batch', () => {
+    const container = document.createElement('div');
+    ReactDOM.render(<span>a</span>, container);
+    ReactDOM.unstable_batchedUpdates(function () {
+      ReactDOM.unmountComponentAtNode(container);
+      ReactDOM.render(<span>b</span>, container);
+    });
+    expect(container.textContent).toBe('b');
+  });
+
+  it('handles reentrant mounting in synchronous mode', () => {
+    let mounts = 0;
+    class Editor extends React.Component {
+      render() {
+        return <div>{this.props.text}</div>;
+      }
+      componentDidMount() {
+        mounts++;
+        // This should be called only once but we guard just in case.
+        if (!this.props.rendered) {
+          this.props.onChange({rendered: true});
+        }
+      }
+    }
+
+    const container = document.createElement('div');
+    function render() {
+      ReactDOM.render(
+        <Editor
+          onChange={newProps => {
+            props = {...props, ...newProps};
+            render();
+          }}
+          {...props}
+        />,
+        container,
+      );
+    }
+
+    let props = {text: 'hello', rendered: false};
+    render();
+    props = {...props, text: 'goodbye'};
+    render();
+    expect(container.textContent).toBe('goodbye');
+    expect(mounts).toBe(1);
+  });
+
+  it('mounts and unmounts are sync even in a batch', () => {
+    const ops = [];
+    const container = document.createElement('div');
+    ReactDOM.unstable_batchedUpdates(() => {
+      ReactDOM.render(<div>Hello</div>, container);
+      ops.push(container.textContent);
+      ReactDOM.unmountComponentAtNode(container);
+      ops.push(container.textContent);
+    });
+    expect(ops).toEqual(['Hello', '']);
+  });
+
+  it(
+    'in legacy mode, updates in componentWillUpdate and componentDidUpdate ' +
+      'should both flush in the immediately subsequent commit',
+    () => {
+      const ops = [];
+      class Foo extends React.Component {
+        state = {a: false, b: false};
+        UNSAFE_componentWillUpdate(_, nextState) {
+          if (!nextState.a) {
+            this.setState({a: true});
+          }
+        }
+        componentDidUpdate() {
+          ops.push('Foo updated');
+          if (!this.state.b) {
+            this.setState({b: true});
+          }
+        }
+        render() {
+          ops.push(`a: ${this.state.a}, b: ${this.state.b}`);
+          return null;
+        }
+      }
+
+      const container = document.createElement('div');
+      // Mount
+      ReactDOM.render(<Foo />, container);
+      // Root update
+      ReactDOM.render(<Foo />, container);
+      expect(ops).toEqual([
+        // Mount
+        'a: false, b: false',
+        // Root update
+        'a: false, b: false',
+        'Foo updated',
+        // Subsequent update (both a and b should have flushed)
+        'a: true, b: true',
+        'Foo updated',
+        // There should not be any additional updates
+      ]);
+    },
+  );
+
+  it(
+    'in legacy mode, updates in componentWillUpdate and componentDidUpdate ' +
+      '(on a sibling) should both flush in the immediately subsequent commit',
+    () => {
+      const ops = [];
+      class Foo extends React.Component {
+        state = {a: false};
+        UNSAFE_componentWillUpdate(_, nextState) {
+          if (!nextState.a) {
+            this.setState({a: true});
+          }
+        }
+        componentDidUpdate() {
+          ops.push('Foo updated');
+        }
+        render() {
+          ops.push(`a: ${this.state.a}`);
+          return null;
+        }
+      }
+
+      class Bar extends React.Component {
+        state = {b: false};
+        componentDidUpdate() {
+          ops.push('Bar updated');
+          if (!this.state.b) {
+            this.setState({b: true});
+          }
+        }
+        render() {
+          ops.push(`b: ${this.state.b}`);
+          return null;
+        }
+      }
+
+      const container = document.createElement('div');
+      // Mount
+      ReactDOM.render(
+        <div>
+          <Foo />
+          <Bar />
+        </div>,
+        container,
+      );
+      // Root update
+      ReactDOM.render(
+        <div>
+          <Foo />
+          <Bar />
+        </div>,
+        container,
+      );
+      expect(ops).toEqual([
+        // Mount
+        'a: false',
+        'b: false',
+        // Root update
+        'a: false',
+        'b: false',
+        'Foo updated',
+        'Bar updated',
+        // Subsequent update (both a and b should have flushed)
+        'a: true',
+        'b: true',
+        'Foo updated',
+        'Bar updated',
+        // There should not be any additional updates
+      ]);
+    },
+  );
+
+  it('uses correct base state for setState inside render phase', () => {
+    const ops = [];
+
+    class Foo extends React.Component {
+      state = {step: 0};
+      render() {
+        const memoizedStep = this.state.step;
+        this.setState(baseState => {
+          const baseStep = baseState.step;
+          ops.push(`base: ${baseStep}, memoized: ${memoizedStep}`);
+          return baseStep === 0 ? {step: 1} : null;
+        });
+        return null;
+      }
+    }
+
+    const container = document.createElement('div');
+    expect(() => ReactDOM.render(<Foo />, container)).toErrorDev(
+      'Cannot update during an existing state transition',
+    );
+    expect(ops).toEqual(['base: 0, memoized: 0', 'base: 1, memoized: 1']);
+  });
+
+  it('does not re-render if state update is null', () => {
+    const container = document.createElement('div');
+
+    let instance;
+    let ops = [];
+    class Foo extends React.Component {
+      render() {
+        instance = this;
+        ops.push('render');
+        return <div />;
+      }
+    }
+    ReactDOM.render(<Foo />, container);
+
+    ops = [];
+    instance.setState(() => null);
+    expect(ops).toEqual([]);
+  });
+
+  // Will change once we switch to async by default
+  it('synchronously renders hidden subtrees', () => {
+    const container = document.createElement('div');
+    let ops = [];
+
+    function Baz() {
+      ops.push('Baz');
+      return null;
+    }
+
+    function Bar() {
+      ops.push('Bar');
+      return null;
+    }
+
+    function Foo() {
+      ops.push('Foo');
+      return (
+        <div>
+          <div hidden={true}>
+            <Bar />
+          </div>
+          <Baz />
+        </div>
+      );
+    }
+
+    // Mount
+    ReactDOM.render(<Foo />, container);
+    expect(ops).toEqual(['Foo', 'Bar', 'Baz']);
+    ops = [];
+
+    // Update
+    ReactDOM.render(<Foo />, container);
+    expect(ops).toEqual(['Foo', 'Bar', 'Baz']);
+  });
+
+  it('can render ridiculously large number of roots without triggering infinite update loop error', () => {
+    class Foo extends React.Component {
+      componentDidMount() {
+        const limit = 1200;
+        for (let i = 0; i < limit; i++) {
+          if (i < limit - 1) {
+            ReactDOM.render(<div />, document.createElement('div'));
+          } else {
+            ReactDOM.render(<div />, document.createElement('div'), () => {
+              // The "nested update limit" error isn't thrown until setState
+              this.setState({});
+            });
+          }
+        }
+      }
+      render() {
+        return null;
+      }
+    }
+
+    const container = document.createElement('div');
+    ReactDOM.render(<Foo />, container);
+  });
+
+  it('resets the update counter for unrelated updates', () => {
+    const container = document.createElement('div');
+    const ref = React.createRef();
+
+    class EventuallyTerminating extends React.Component {
+      state = {step: 0};
+      componentDidMount() {
+        this.setState({step: 1});
+      }
+      componentDidUpdate() {
+        if (this.state.step < limit) {
+          this.setState({step: this.state.step + 1});
+        }
+      }
+      render() {
+        return this.state.step;
+      }
+    }
+
+    let limit = 55;
+    expect(() => {
+      ReactDOM.render(<EventuallyTerminating ref={ref} />, container);
+    }).toThrow('Maximum');
+
+    // Verify that we don't go over the limit if these updates are unrelated.
+    limit -= 10;
+    ReactDOM.render(<EventuallyTerminating ref={ref} />, container);
+    expect(container.textContent).toBe(limit.toString());
+    ref.current.setState({step: 0});
+    expect(container.textContent).toBe(limit.toString());
+    ref.current.setState({step: 0});
+    expect(container.textContent).toBe(limit.toString());
+
+    limit += 10;
+    expect(() => {
+      ref.current.setState({step: 0});
+    }).toThrow('Maximum');
+    expect(ref.current).toBe(null);
+  });
+
+  it('does not fall into an infinite update loop', () => {
+    class NonTerminating extends React.Component {
+      state = {step: 0};
+      componentDidMount() {
+        this.setState({step: 1});
+      }
+      UNSAFE_componentWillUpdate() {
+        this.setState({step: 2});
+      }
+      render() {
+        return (
+          <div>
+            Hello {this.props.name}
+            {this.state.step}
+          </div>
+        );
+      }
+    }
+
+    const container = document.createElement('div');
+    expect(() => {
+      ReactDOM.render(<NonTerminating />, container);
+    }).toThrow('Maximum');
+  });
+
+  it('does not fall into an infinite update loop with useLayoutEffect', () => {
+    function NonTerminating() {
+      const [step, setStep] = React.useState(0);
+      React.useLayoutEffect(() => {
+        setStep(x => x + 1);
+      });
+      return step;
+    }
+
+    const container = document.createElement('div');
+    expect(() => {
+      ReactDOM.render(<NonTerminating />, container);
+    }).toThrow('Maximum');
+  });
+
+  it('can recover after falling into an infinite update loop', () => {
+    class NonTerminating extends React.Component {
+      state = {step: 0};
+      componentDidMount() {
+        this.setState({step: 1});
+      }
+      componentDidUpdate() {
+        this.setState({step: 2});
+      }
+      render() {
+        return this.state.step;
+      }
+    }
+
+    class Terminating extends React.Component {
+      state = {step: 0};
+      componentDidMount() {
+        this.setState({step: 1});
+      }
+      render() {
+        return this.state.step;
+      }
+    }
+
+    const container = document.createElement('div');
+    expect(() => {
+      ReactDOM.render(<NonTerminating />, container);
+    }).toThrow('Maximum');
+
+    ReactDOM.render(<Terminating />, container);
+    expect(container.textContent).toBe('1');
+
+    expect(() => {
+      ReactDOM.render(<NonTerminating />, container);
+    }).toThrow('Maximum');
+
+    ReactDOM.render(<Terminating />, container);
+    expect(container.textContent).toBe('1');
+  });
+
+  it('does not fall into mutually recursive infinite update loop with same container', () => {
+    // Note: this test would fail if there were two or more different roots.
+
+    class A extends React.Component {
+      componentDidMount() {
+        ReactDOM.render(<B />, container);
+      }
+      render() {
+        return null;
+      }
+    }
+
+    class B extends React.Component {
+      componentDidMount() {
+        ReactDOM.render(<A />, container);
+      }
+      render() {
+        return null;
+      }
+    }
+
+    const container = document.createElement('div');
+    expect(() => {
+      ReactDOM.render(<A />, container);
+    }).toThrow('Maximum');
+  });
+
+  it('does not fall into an infinite error loop', () => {
+    function BadRender() {
+      throw new Error('error');
+    }
+
+    class ErrorBoundary extends React.Component {
+      componentDidCatch() {
+        // Schedule a no-op state update to avoid triggering a DEV warning in the test.
+        this.setState({});
+
+        this.props.parent.remount();
+      }
+      render() {
+        return <BadRender />;
+      }
+    }
+
+    class NonTerminating extends React.Component {
+      state = {step: 0};
+      remount() {
+        this.setState(state => ({step: state.step + 1}));
+      }
+      render() {
+        return <ErrorBoundary key={this.state.step} parent={this} />;
+      }
+    }
+
+    const container = document.createElement('div');
+    expect(() => {
+      ReactDOM.render(<NonTerminating />, container);
+    }).toThrow('Maximum');
+  });
+
+  it('can schedule ridiculously many updates within the same batch without triggering a maximum update error', () => {
+    const subscribers = [];
+
+    class Child extends React.Component {
+      state = {value: 'initial'};
+      componentDidMount() {
+        subscribers.push(this);
+      }
+      render() {
+        return null;
+      }
+    }
+
+    class App extends React.Component {
+      render() {
+        const children = [];
+        for (let i = 0; i < 1200; i++) {
+          children.push(<Child key={i} />);
+        }
+        return children;
+      }
+    }
+
+    const container = document.createElement('div');
+    ReactDOM.render(<App />, container);
+
+    ReactDOM.unstable_batchedUpdates(() => {
+      subscribers.forEach(s => {
+        s.setState({value: 'update'});
+      });
+    });
+  });
+
+  // TODO: Replace this branch with @gate pragmas
+  if (__DEV__) {
+    it('can have nested updates if they do not cross the limit', async () => {
+      let _setStep;
+      const LIMIT = 50;
+
+      function Terminating() {
+        const [step, setStep] = React.useState(0);
+        _setStep = setStep;
+        React.useEffect(() => {
+          if (step < LIMIT) {
+            setStep(x => x + 1);
+          }
+        });
+        Scheduler.log(step);
+        return step;
+      }
+
+      const container = document.createElement('div');
+      await act(() => {
+        ReactDOM.render(<Terminating />, container);
+      });
+      expect(container.textContent).toBe('50');
+      await act(() => {
+        _setStep(0);
+      });
+      expect(container.textContent).toBe('50');
+    });
+
+    it('can have many updates inside useEffect without triggering a warning', async () => {
+      function Terminating() {
+        const [step, setStep] = React.useState(0);
+        React.useEffect(() => {
+          for (let i = 0; i < 1000; i++) {
+            setStep(x => x + 1);
+          }
+          Scheduler.log('Done');
+        }, []);
+        return step;
+      }
+
+      const container = document.createElement('div');
+      await act(() => {
+        ReactDOM.render(<Terminating />, container);
+      });
+
+      assertLog(['Done']);
+      expect(container.textContent).toBe('1000');
+    });
+  }
+});
diff --git a/packages/react-dom/src/__tests__/ReactUpdates-test.js b/packages/react-dom/src/__tests__/ReactUpdates-test.js
index 67abeb6dd0ad6..b460e9247646f 100644
--- a/packages/react-dom/src/__tests__/ReactUpdates-test.js
+++ b/packages/react-dom/src/__tests__/ReactUpdates-test.js
@@ -48,226 +48,283 @@ describe('ReactUpdates', () => {
     );
   }
 
-  it('should batch state when updating state twice', () => {
-    let updateCount = 0;
-
-    class Component extends React.Component {
-      state = {x: 0};
-
-      componentDidUpdate() {
-        updateCount++;
-      }
+  it('should batch state when updating state twice', async () => {
+    let componentState;
+    let setState;
+
+    function Component() {
+      const [state, _setState] = React.useState(0);
+      componentState = state;
+      setState = _setState;
+      React.useLayoutEffect(() => {
+        Scheduler.log('Commit');
+      });
 
-      render() {
-        return <div>{this.state.x}</div>;
-      }
+      return <div>{state}</div>;
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Component />);
-    expect(instance.state.x).toBe(0);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component />);
+    });
 
-    ReactDOM.unstable_batchedUpdates(function () {
-      instance.setState({x: 1});
-      instance.setState({x: 2});
-      expect(instance.state.x).toBe(0);
-      expect(updateCount).toBe(0);
+    assertLog(['Commit']);
+    expect(container.firstChild.textContent).toBe('0');
+
+    await act(() => {
+      setState(1);
+      setState(2);
+      expect(componentState).toBe(0);
+      expect(container.firstChild.textContent).toBe('0');
+      assertLog([]);
     });
 
-    expect(instance.state.x).toBe(2);
-    expect(updateCount).toBe(1);
+    expect(componentState).toBe(2);
+    assertLog(['Commit']);
+    expect(container.firstChild.textContent).toBe('2');
   });
 
-  it('should batch state when updating two different state keys', () => {
-    let updateCount = 0;
+  it('should batch state when updating two different states', async () => {
+    let componentStateA;
+    let componentStateB;
+    let setStateA;
+    let setStateB;
 
-    class Component extends React.Component {
-      state = {x: 0, y: 0};
+    function Component() {
+      const [stateA, _setStateA] = React.useState(0);
+      const [stateB, _setStateB] = React.useState(0);
+      componentStateA = stateA;
+      componentStateB = stateB;
+      setStateA = _setStateA;
+      setStateB = _setStateB;
 
-      componentDidUpdate() {
-        updateCount++;
-      }
+      React.useLayoutEffect(() => {
+        Scheduler.log('Commit');
+      });
 
-      render() {
-        return (
-          <div>
-            ({this.state.x}, {this.state.y})
-          </div>
-        );
-      }
+      return (
+        <div>
+          {stateA} {stateB}
+        </div>
+      );
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Component />);
-    expect(instance.state.x).toBe(0);
-    expect(instance.state.y).toBe(0);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component />);
+    });
 
-    ReactDOM.unstable_batchedUpdates(function () {
-      instance.setState({x: 1});
-      instance.setState({y: 2});
-      expect(instance.state.x).toBe(0);
-      expect(instance.state.y).toBe(0);
-      expect(updateCount).toBe(0);
+    assertLog(['Commit']);
+    expect(container.firstChild.textContent).toBe('0 0');
+
+    await act(() => {
+      setStateA(1);
+      setStateB(2);
+      expect(componentStateA).toBe(0);
+      expect(componentStateB).toBe(0);
+      expect(container.firstChild.textContent).toBe('0 0');
+      assertLog([]);
     });
 
-    expect(instance.state.x).toBe(1);
-    expect(instance.state.y).toBe(2);
-    expect(updateCount).toBe(1);
+    expect(componentStateA).toBe(1);
+    expect(componentStateB).toBe(2);
+    assertLog(['Commit']);
+    expect(container.firstChild.textContent).toBe('1 2');
   });
 
-  it('should batch state and props together', () => {
-    let updateCount = 0;
+  it('should batch state and props together', async () => {
+    let setState;
+    let componentProp;
+    let componentState;
 
-    class Component extends React.Component {
-      state = {y: 0};
+    function Component({prop}) {
+      const [state, _setState] = React.useState(0);
+      componentProp = prop;
+      componentState = state;
+      setState = _setState;
 
-      componentDidUpdate() {
-        updateCount++;
-      }
+      React.useLayoutEffect(() => {
+        Scheduler.log('Commit');
+      });
 
-      render() {
-        return (
-          <div>
-            ({this.props.x}, {this.state.y})
-          </div>
-        );
-      }
+      return (
+        <div>
+          {prop} {state}
+        </div>
+      );
     }
 
     const container = document.createElement('div');
-    const instance = ReactDOM.render(<Component x={0} />, container);
-    expect(instance.props.x).toBe(0);
-    expect(instance.state.y).toBe(0);
-
-    ReactDOM.unstable_batchedUpdates(function () {
-      ReactDOM.render(<Component x={1} />, container);
-      instance.setState({y: 2});
-      expect(instance.props.x).toBe(0);
-      expect(instance.state.y).toBe(0);
-      expect(updateCount).toBe(0);
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component prop={0} />);
+    });
+
+    assertLog(['Commit']);
+    expect(container.firstChild.textContent).toBe('0 0');
+
+    await act(() => {
+      root.render(<Component prop={1} />);
+      setState(2);
+      expect(componentProp).toBe(0);
+      expect(componentState).toBe(0);
+      expect(container.firstChild.textContent).toBe('0 0');
+      assertLog([]);
     });
 
-    expect(instance.props.x).toBe(1);
-    expect(instance.state.y).toBe(2);
-    expect(updateCount).toBe(1);
+    expect(componentProp).toBe(1);
+    expect(componentState).toBe(2);
+    assertLog(['Commit']);
+    expect(container.firstChild.textContent).toBe('1 2');
   });
 
-  it('should batch parent/child state updates together', () => {
-    let parentUpdateCount = 0;
+  it('should batch parent/child state updates together', async () => {
+    let childRef;
+    let parentState;
+    let childState;
+    let setParentState;
+    let setChildState;
 
-    class Parent extends React.Component {
-      state = {x: 0};
-      childRef = React.createRef();
+    function Parent() {
+      const [state, _setState] = React.useState(0);
+      parentState = state;
+      setParentState = _setState;
 
-      componentDidUpdate() {
-        parentUpdateCount++;
-      }
+      React.useLayoutEffect(() => {
+        Scheduler.log('Parent Commit');
+      });
 
-      render() {
-        return (
-          <div>
-            <Child ref={this.childRef} x={this.state.x} />
-          </div>
-        );
-      }
+      return (
+        <div>
+          <Child prop={state} />
+        </div>
+      );
     }
 
-    let childUpdateCount = 0;
-
-    class Child extends React.Component {
-      state = {y: 0};
+    function Child({prop}) {
+      const [state, _setState] = React.useState(0);
+      childState = state;
+      setChildState = _setState;
 
-      componentDidUpdate() {
-        childUpdateCount++;
-      }
+      React.useLayoutEffect(() => {
+        Scheduler.log('Child Commit');
+      });
 
-      render() {
-        return <div>{this.props.x + this.state.y}</div>;
-      }
+      return (
+        <div
+          ref={ref => {
+            childRef = ref;
+          }}>
+          {prop} {state}
+        </div>
+      );
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Parent />);
-    const child = instance.childRef.current;
-    expect(instance.state.x).toBe(0);
-    expect(child.state.y).toBe(0);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Parent />);
+    });
 
-    ReactDOM.unstable_batchedUpdates(function () {
-      instance.setState({x: 1});
-      child.setState({y: 2});
-      expect(instance.state.x).toBe(0);
-      expect(child.state.y).toBe(0);
-      expect(parentUpdateCount).toBe(0);
-      expect(childUpdateCount).toBe(0);
+    assertLog(['Child Commit', 'Parent Commit']);
+    expect(childRef.textContent).toBe('0 0');
+
+    await act(() => {
+      // Parent update first.
+      setParentState(1);
+      setChildState(2);
+      expect(parentState).toBe(0);
+      expect(childState).toBe(0);
+      expect(childRef.textContent).toBe('0 0');
+      assertLog([]);
     });
 
-    expect(instance.state.x).toBe(1);
-    expect(child.state.y).toBe(2);
-    expect(parentUpdateCount).toBe(1);
-    expect(childUpdateCount).toBe(1);
+    expect(parentState).toBe(1);
+    expect(childState).toBe(2);
+    expect(childRef.textContent).toBe('1 2');
+    assertLog(['Child Commit', 'Parent Commit']);
   });
 
-  it('should batch child/parent state updates together', () => {
-    let parentUpdateCount = 0;
+  it('should batch child/parent state updates together', async () => {
+    let childRef;
+    let parentState;
+    let childState;
+    let setParentState;
+    let setChildState;
 
-    class Parent extends React.Component {
-      state = {x: 0};
-      childRef = React.createRef();
+    function Parent() {
+      const [state, _setState] = React.useState(0);
+      parentState = state;
+      setParentState = _setState;
 
-      componentDidUpdate() {
-        parentUpdateCount++;
-      }
+      React.useLayoutEffect(() => {
+        Scheduler.log('Parent Commit');
+      });
 
-      render() {
-        return (
-          <div>
-            <Child ref={this.childRef} x={this.state.x} />
-          </div>
-        );
-      }
+      return (
+        <div>
+          <Child prop={state} />
+        </div>
+      );
     }
 
-    let childUpdateCount = 0;
+    function Child({prop}) {
+      const [state, _setState] = React.useState(0);
+      childState = state;
+      setChildState = _setState;
 
-    class Child extends React.Component {
-      state = {y: 0};
-
-      componentDidUpdate() {
-        childUpdateCount++;
-      }
+      React.useLayoutEffect(() => {
+        Scheduler.log('Child Commit');
+      });
 
-      render() {
-        return <div>{this.props.x + this.state.y}</div>;
-      }
+      return (
+        <div
+          ref={ref => {
+            childRef = ref;
+          }}>
+          {prop} {state}
+        </div>
+      );
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Parent />);
-    const child = instance.childRef.current;
-    expect(instance.state.x).toBe(0);
-    expect(child.state.y).toBe(0);
-
-    ReactDOM.unstable_batchedUpdates(function () {
-      child.setState({y: 2});
-      instance.setState({x: 1});
-      expect(instance.state.x).toBe(0);
-      expect(child.state.y).toBe(0);
-      expect(parentUpdateCount).toBe(0);
-      expect(childUpdateCount).toBe(0);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Parent />);
     });
 
-    expect(instance.state.x).toBe(1);
-    expect(child.state.y).toBe(2);
-    expect(parentUpdateCount).toBe(1);
+    assertLog(['Child Commit', 'Parent Commit']);
+    expect(childRef.textContent).toBe('0 0');
+
+    await act(() => {
+      // Child update first.
+      setChildState(2);
+      setParentState(1);
+      expect(parentState).toBe(0);
+      expect(childState).toBe(0);
+      expect(childRef.textContent).toBe('0 0');
+      assertLog([]);
+    });
 
-    // Batching reduces the number of updates here to 1.
-    expect(childUpdateCount).toBe(1);
+    expect(parentState).toBe(1);
+    expect(childState).toBe(2);
+    expect(childRef.textContent).toBe('1 2');
+    assertLog(['Child Commit', 'Parent Commit']);
   });
 
-  it('should support chained state updates', () => {
-    let updateCount = 0;
-
+  it('should support chained state updates', async () => {
+    let instance;
     class Component extends React.Component {
       state = {x: 0};
+      constructor(props) {
+        super(props);
+        instance = this;
+      }
 
       componentDidUpdate() {
-        updateCount++;
+        Scheduler.log('Update');
       }
 
       render() {
@@ -275,43 +332,55 @@ describe('ReactUpdates', () => {
       }
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component />);
+    });
+
     expect(instance.state.x).toBe(0);
+    expect(container.firstChild.textContent).toBe('0');
 
     let innerCallbackRun = false;
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       instance.setState({x: 1}, function () {
         instance.setState({x: 2}, function () {
-          expect(this).toBe(instance);
           innerCallbackRun = true;
           expect(instance.state.x).toBe(2);
-          expect(updateCount).toBe(2);
+          expect(container.firstChild.textContent).toBe('2');
+          assertLog(['Update']);
         });
         expect(instance.state.x).toBe(1);
-        expect(updateCount).toBe(1);
+        expect(container.firstChild.textContent).toBe('1');
+        assertLog(['Update']);
       });
       expect(instance.state.x).toBe(0);
-      expect(updateCount).toBe(0);
+      expect(container.firstChild.textContent).toBe('0');
+      assertLog([]);
     });
 
-    expect(innerCallbackRun).toBeTruthy();
+    assertLog([]);
     expect(instance.state.x).toBe(2);
-    expect(updateCount).toBe(2);
+    expect(innerCallbackRun).toBeTruthy();
+    expect(container.firstChild.textContent).toBe('2');
   });
 
-  it('should batch forceUpdate together', () => {
+  it('should batch forceUpdate together', async () => {
+    let instance;
     let shouldUpdateCount = 0;
-    let updateCount = 0;
-
     class Component extends React.Component {
       state = {x: 0};
 
+      constructor(props) {
+        super(props);
+        instance = this;
+      }
       shouldComponentUpdate() {
         shouldUpdateCount++;
       }
 
       componentDidUpdate() {
-        updateCount++;
+        Scheduler.log('Update');
       }
 
       render() {
@@ -319,80 +388,82 @@ describe('ReactUpdates', () => {
       }
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Component />);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component />);
+    });
+
+    assertLog([]);
     expect(instance.state.x).toBe(0);
 
-    let callbacksRun = 0;
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       instance.setState({x: 1}, function () {
-        callbacksRun++;
+        Scheduler.log('callback');
       });
       instance.forceUpdate(function () {
-        callbacksRun++;
+        Scheduler.log('forceUpdate');
       });
+      assertLog([]);
       expect(instance.state.x).toBe(0);
-      expect(updateCount).toBe(0);
+      expect(container.firstChild.textContent).toBe('0');
     });
 
-    expect(callbacksRun).toBe(2);
     // shouldComponentUpdate shouldn't be called since we're forcing
     expect(shouldUpdateCount).toBe(0);
+    assertLog(['Update', 'callback', 'forceUpdate']);
     expect(instance.state.x).toBe(1);
-    expect(updateCount).toBe(1);
+    expect(container.firstChild.textContent).toBe('1');
   });
 
-  it('should update children even if parent blocks updates', () => {
-    let parentRenderCount = 0;
-    let childRenderCount = 0;
-
+  it('should update children even if parent blocks updates', async () => {
+    let instance;
     class Parent extends React.Component {
       childRef = React.createRef();
 
+      constructor(props) {
+        super(props);
+        instance = this;
+      }
       shouldComponentUpdate() {
         return false;
       }
 
       render() {
-        parentRenderCount++;
+        Scheduler.log('Parent render');
         return <Child ref={this.childRef} />;
       }
     }
 
     class Child extends React.Component {
       render() {
-        childRenderCount++;
+        Scheduler.log('Child render');
         return <div />;
       }
     }
 
-    expect(parentRenderCount).toBe(0);
-    expect(childRenderCount).toBe(0);
-
-    let instance = <Parent />;
-    instance = ReactTestUtils.renderIntoDocument(instance);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Parent />);
+    });
 
-    expect(parentRenderCount).toBe(1);
-    expect(childRenderCount).toBe(1);
+    assertLog(['Parent render', 'Child render']);
 
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       instance.setState({x: 1});
     });
 
-    expect(parentRenderCount).toBe(1);
-    expect(childRenderCount).toBe(1);
+    assertLog([]);
 
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       instance.childRef.current.setState({x: 1});
     });
 
-    expect(parentRenderCount).toBe(1);
-    expect(childRenderCount).toBe(2);
+    assertLog(['Child render']);
   });
 
-  it('should not reconcile children passed via props', () => {
-    let numMiddleRenders = 0;
-    let numBottomRenders = 0;
-
+  it('should not reconcile children passed via props', async () => {
     class Top extends React.Component {
       render() {
         return (
@@ -409,26 +480,31 @@ describe('ReactUpdates', () => {
       }
 
       render() {
-        numMiddleRenders++;
+        Scheduler.log('Middle');
         return React.Children.only(this.props.children);
       }
     }
 
     class Bottom extends React.Component {
       render() {
-        numBottomRenders++;
+        Scheduler.log('Bottom');
         return null;
       }
     }
 
-    ReactTestUtils.renderIntoDocument(<Top />);
-    expect(numMiddleRenders).toBe(2);
-    expect(numBottomRenders).toBe(1);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Top />);
+    });
+
+    assertLog(['Middle', 'Bottom', 'Middle']);
   });
 
-  it('should flow updates correctly', () => {
+  it('should flow updates correctly', async () => {
     let willUpdates = [];
     let didUpdates = [];
+    let instance;
 
     const UpdateLoggingMixin = {
       UNSAFE_componentWillUpdate: function () {
@@ -482,7 +558,10 @@ describe('ReactUpdates', () => {
     class App extends React.Component {
       switcherRef = React.createRef();
       childRef = React.createRef();
-
+      constructor(props) {
+        super(props);
+        instance = this;
+      }
       render() {
         return (
           <Switcher ref={this.switcherRef}>
@@ -493,8 +572,10 @@ describe('ReactUpdates', () => {
     }
     Object.assign(App.prototype, UpdateLoggingMixin);
 
-    let root = <App />;
-    root = ReactTestUtils.renderIntoDocument(root);
+    const container = document.createElement('div');
+    await act(() => {
+      ReactDOMClient.createRoot(container).render(<App />);
+    });
 
     function expectUpdates(desiredWillUpdates, desiredDidUpdates) {
       let i;
@@ -512,10 +593,14 @@ describe('ReactUpdates', () => {
       c.setState({x: 1});
     }
 
-    function testUpdates(components, desiredWillUpdates, desiredDidUpdates) {
+    async function testUpdates(
+      components,
+      desiredWillUpdates,
+      desiredDidUpdates,
+    ) {
       let i;
 
-      ReactDOM.unstable_batchedUpdates(function () {
+      await act(() => {
         for (i = 0; i < components.length; i++) {
           triggerUpdate(components[i]);
         }
@@ -525,7 +610,7 @@ describe('ReactUpdates', () => {
 
       // Try them in reverse order
 
-      ReactDOM.unstable_batchedUpdates(function () {
+      await act(() => {
         for (i = components.length - 1; i >= 0; i--) {
           triggerUpdate(components[i]);
         }
@@ -533,42 +618,48 @@ describe('ReactUpdates', () => {
 
       expectUpdates(desiredWillUpdates, desiredDidUpdates);
     }
-    testUpdates(
-      [root.switcherRef.current.boxRef.current, root.switcherRef.current],
+    await testUpdates(
+      [
+        instance.switcherRef.current.boxRef.current,
+        instance.switcherRef.current,
+      ],
       // Owner-child relationships have inverse will and did
       ['Switcher', 'Box'],
       ['Box', 'Switcher'],
     );
 
-    testUpdates(
-      [root.childRef.current, root.switcherRef.current.boxRef.current],
+    await testUpdates(
+      [instance.childRef.current, instance.switcherRef.current.boxRef.current],
       // Not owner-child so reconcile independently
       ['Box', 'Child'],
       ['Box', 'Child'],
     );
 
-    testUpdates(
-      [root.childRef.current, root.switcherRef.current],
+    await testUpdates(
+      [instance.childRef.current, instance.switcherRef.current],
       // Switcher owns Box and Child, Box does not own Child
       ['Switcher', 'Box', 'Child'],
       ['Box', 'Switcher', 'Child'],
     );
   });
 
-  it('should queue mount-ready handlers across different roots', () => {
+  it('should queue mount-ready handlers across different roots', async () => {
     // We'll define two components A and B, then update both of them. When A's
     // componentDidUpdate handlers is called, B's DOM should already have been
     // updated.
 
     const bContainer = document.createElement('div');
-
+    let a;
     let b;
 
     let aUpdated = false;
 
     class A extends React.Component {
       state = {x: 0};
-
+      constructor(props) {
+        super(props);
+        a = this;
+      }
       componentDidUpdate() {
         expect(ReactDOM.findDOMNode(b).textContent).toBe('B1');
         aUpdated = true;
@@ -576,7 +667,6 @@ describe('ReactUpdates', () => {
 
       render() {
         let portal = null;
-        // If we're using Fiber, we use Portals instead to achieve this.
         portal = ReactDOM.createPortal(<B ref={n => (b = n)} />, bContainer);
         return (
           <div>
@@ -595,8 +685,13 @@ describe('ReactUpdates', () => {
       }
     }
 
-    const a = ReactTestUtils.renderIntoDocument(<A />);
-    ReactDOM.unstable_batchedUpdates(function () {
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<A />);
+    });
+
+    await act(() => {
       a.setState({x: 1});
       b.setState({x: 1});
     });
@@ -604,13 +699,16 @@ describe('ReactUpdates', () => {
     expect(aUpdated).toBe(true);
   });
 
-  it('should flush updates in the correct order', () => {
+  it('should flush updates in the correct order', async () => {
     const updates = [];
-
+    let instance;
     class Outer extends React.Component {
       state = {x: 0};
       innerRef = React.createRef();
-
+      constructor(props) {
+        super(props);
+        instance = this;
+      }
       render() {
         updates.push('Outer-render-' + this.state.x);
         return (
@@ -643,14 +741,20 @@ describe('ReactUpdates', () => {
       }
     }
 
-    const instance = ReactTestUtils.renderIntoDocument(<Outer />);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Outer />);
+    });
 
-    updates.push('Outer-setState-1');
-    instance.setState({x: 1}, function () {
-      updates.push('Outer-callback-1');
-      updates.push('Outer-setState-2');
-      instance.setState({x: 2}, function () {
-        updates.push('Outer-callback-2');
+    await act(() => {
+      updates.push('Outer-setState-1');
+      instance.setState({x: 1}, function () {
+        updates.push('Outer-callback-1');
+        updates.push('Outer-setState-2');
+        instance.setState({x: 2}, function () {
+          updates.push('Outer-callback-2');
+        });
       });
     });
 
@@ -686,7 +790,7 @@ describe('ReactUpdates', () => {
     /* eslint-enable indent */
   });
 
-  it('should flush updates in the correct order across roots', () => {
+  it('should flush updates in the correct order across roots', async () => {
     const instances = [];
     const updates = [];
 
@@ -699,22 +803,26 @@ describe('ReactUpdates', () => {
       componentDidMount() {
         instances.push(this);
         if (this.props.depth < this.props.count) {
-          ReactDOM.render(
+          const root = ReactDOMClient.createRoot(ReactDOM.findDOMNode(this));
+          root.render(
             <MockComponent
               depth={this.props.depth + 1}
               count={this.props.count}
             />,
-            ReactDOM.findDOMNode(this),
           );
         }
       }
     }
 
-    ReactTestUtils.renderIntoDocument(<MockComponent depth={0} count={2} />);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<MockComponent depth={0} count={2} />);
+    });
 
     expect(updates).toEqual([0, 1, 2]);
 
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       // Simulate update on each component from top to bottom.
       instances.forEach(function (instance) {
         instance.forceUpdate();
@@ -777,7 +885,7 @@ describe('ReactUpdates', () => {
     expect(ReactDOM.findDOMNode(x).textContent).toBe('1');
   });
 
-  it('should queue updates from during mount', () => {
+  it('should queue updates from during mount', async () => {
     // See https://github.com/facebook/react/issues/1353
     let a;
 
@@ -803,8 +911,11 @@ describe('ReactUpdates', () => {
       }
     }
 
-    ReactDOM.unstable_batchedUpdates(function () {
-      ReactTestUtils.renderIntoDocument(
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+
+    await act(() => {
+      root.render(
         <div>
           <A />
           <B />
@@ -812,13 +923,10 @@ describe('ReactUpdates', () => {
       );
     });
 
-    expect(a.state.x).toBe(1);
-    expect(ReactDOM.findDOMNode(a).textContent).toBe('A1');
+    expect(container.firstChild.textContent).toBe('A1');
   });
 
-  it('calls componentWillReceiveProps setState callback properly', () => {
-    let callbackCount = 0;
-
+  it('calls componentWillReceiveProps setState callback properly', async () => {
     class A extends React.Component {
       state = {x: this.props.x};
 
@@ -827,7 +935,7 @@ describe('ReactUpdates', () => {
         this.setState({x: newX}, function () {
           // State should have updated by the time this callback gets called
           expect(this.state.x).toBe(newX);
-          callbackCount++;
+          Scheduler.log('Callback');
         });
       }
 
@@ -837,13 +945,22 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
-    ReactDOM.render(<A x={1} />, container);
-    ReactDOM.render(<A x={2} />, container);
-    expect(callbackCount).toBe(1);
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<A x={1} />);
+    });
+    assertLog([]);
+
+    // Needs to be a separate act, or it will be batched.
+    await act(() => {
+      root.render(<A x={2} />);
+    });
+
+    assertLog(['Callback']);
   });
 
-  it('does not call render after a component as been deleted', () => {
-    let renderCount = 0;
+  it('does not call render after a component as been deleted', async () => {
+    let componentA = null;
     let componentB = null;
 
     class B extends React.Component {
@@ -854,7 +971,7 @@ describe('ReactUpdates', () => {
       }
 
       render() {
-        renderCount++;
+        Scheduler.log('B');
         return <div />;
       }
     }
@@ -862,21 +979,29 @@ describe('ReactUpdates', () => {
     class A extends React.Component {
       state = {showB: true};
 
+      componentDidMount() {
+        componentA = this;
+      }
       render() {
         return this.state.showB ? <B /> : <div />;
       }
     }
 
-    const component = ReactTestUtils.renderIntoDocument(<A />);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<A />);
+    });
+    assertLog(['B']);
 
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       // B will have scheduled an update but the batching should ensure that its
       // update never fires.
       componentB.setState({updates: 1});
-      component.setState({showB: false});
+      componentA.setState({showB: false});
     });
 
-    expect(renderCount).toBe(1);
+    assertLog([]);
   });
 
   it('throws in setState if the update callback is not a function', () => {
@@ -965,10 +1090,14 @@ describe('ReactUpdates', () => {
     );
   });
 
-  it('does not update one component twice in a batch (#2410)', () => {
+  it('does not update one component twice in a batch (#2410)', async () => {
+    let parent;
     class Parent extends React.Component {
       childRef = React.createRef();
 
+      componentDidMount() {
+        parent = this;
+      }
       getChild = () => {
         return this.childRef.current;
       };
@@ -1009,15 +1138,22 @@ describe('ReactUpdates', () => {
       }
     }
 
-    const parent = ReactTestUtils.renderIntoDocument(<Parent />);
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Parent />);
+    });
+
     const child = parent.getChild();
-    ReactDOM.unstable_batchedUpdates(function () {
+    await act(() => {
       parent.forceUpdate();
       child.forceUpdate();
     });
+
+    expect.assertions(6);
   });
 
-  it('does not update one component twice in a batch (#6371)', () => {
+  it('does not update one component twice in a batch (#6371)', async () => {
     let callbacks = [];
     function emitChange() {
       callbacks.forEach(c => c());
@@ -1064,34 +1200,23 @@ describe('ReactUpdates', () => {
       }
     }
 
-    ReactDOM.render(<App />, document.createElement('div'));
-  });
-
-  it('unstable_batchedUpdates should return value from a callback', () => {
-    const result = ReactDOM.unstable_batchedUpdates(function () {
-      return 42;
+    const root = ReactDOMClient.createRoot(document.createElement('div'));
+    await act(() => {
+      root.render(<App />);
     });
-    expect(result).toEqual(42);
-  });
 
-  it('unmounts and remounts a root in the same batch', () => {
-    const container = document.createElement('div');
-    ReactDOM.render(<span>a</span>, container);
-    ReactDOM.unstable_batchedUpdates(function () {
-      ReactDOM.unmountComponentAtNode(container);
-      ReactDOM.render(<span>b</span>, container);
-    });
-    expect(container.textContent).toBe('b');
+    // Error should not be thrown.
+    expect(true).toBe(true);
   });
 
-  it('handles reentrant mounting in synchronous mode', () => {
-    let mounts = 0;
+  it('handles reentrant mounting in synchronous mode', async () => {
+    let onChangeCalled = false;
     class Editor extends React.Component {
       render() {
         return <div>{this.props.text}</div>;
       }
       componentDidMount() {
-        mounts++;
+        Scheduler.log('Mount');
         // This should be called only once but we guard just in case.
         if (!this.props.rendered) {
           this.props.onChange({rendered: true});
@@ -1100,163 +1225,57 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
     function render() {
-      ReactDOM.render(
+      root.render(
         <Editor
           onChange={newProps => {
+            onChangeCalled = true;
             props = {...props, ...newProps};
             render();
           }}
           {...props}
         />,
-        container,
       );
     }
 
     let props = {text: 'hello', rendered: false};
-    render();
+    await act(() => {
+      render();
+    });
+    assertLog(['Mount']);
     props = {...props, text: 'goodbye'};
-    render();
+    await act(() => {
+      render();
+    });
+
+    assertLog([]);
     expect(container.textContent).toBe('goodbye');
-    expect(mounts).toBe(1);
+    expect(onChangeCalled).toBeTruthy();
   });
 
-  it('mounts and unmounts are sync even in a batch', () => {
-    const ops = [];
+  it('mounts and unmounts are batched', async () => {
     const container = document.createElement('div');
-    ReactDOM.unstable_batchedUpdates(() => {
-      ReactDOM.render(<div>Hello</div>, container);
-      ops.push(container.textContent);
-      ReactDOM.unmountComponentAtNode(container);
-      ops.push(container.textContent);
-    });
-    expect(ops).toEqual(['Hello', '']);
-  });
-
-  it(
-    'in legacy mode, updates in componentWillUpdate and componentDidUpdate ' +
-      'should both flush in the immediately subsequent commit',
-    () => {
-      const ops = [];
-      class Foo extends React.Component {
-        state = {a: false, b: false};
-        UNSAFE_componentWillUpdate(_, nextState) {
-          if (!nextState.a) {
-            this.setState({a: true});
-          }
-        }
-        componentDidUpdate() {
-          ops.push('Foo updated');
-          if (!this.state.b) {
-            this.setState({b: true});
-          }
-        }
-        render() {
-          ops.push(`a: ${this.state.a}, b: ${this.state.b}`);
-          return null;
-        }
-      }
-
-      const container = document.createElement('div');
-      // Mount
-      ReactDOM.render(<Foo />, container);
-      // Root update
-      ReactDOM.render(<Foo />, container);
-      expect(ops).toEqual([
-        // Mount
-        'a: false, b: false',
-        // Root update
-        'a: false, b: false',
-        'Foo updated',
-        // Subsequent update (both a and b should have flushed)
-        'a: true, b: true',
-        'Foo updated',
-        // There should not be any additional updates
-      ]);
-    },
-  );
-
-  it(
-    'in legacy mode, updates in componentWillUpdate and componentDidUpdate ' +
-      '(on a sibling) should both flush in the immediately subsequent commit',
-    () => {
-      const ops = [];
-      class Foo extends React.Component {
-        state = {a: false};
-        UNSAFE_componentWillUpdate(_, nextState) {
-          if (!nextState.a) {
-            this.setState({a: true});
-          }
-        }
-        componentDidUpdate() {
-          ops.push('Foo updated');
-        }
-        render() {
-          ops.push(`a: ${this.state.a}`);
-          return null;
-        }
-      }
+    const root = ReactDOMClient.createRoot(container);
 
-      class Bar extends React.Component {
-        state = {b: false};
-        componentDidUpdate() {
-          ops.push('Bar updated');
-          if (!this.state.b) {
-            this.setState({b: true});
-          }
-        }
-        render() {
-          ops.push(`b: ${this.state.b}`);
-          return null;
-        }
-      }
+    await act(() => {
+      root.render(<div>Hello</div>);
+      expect(container.textContent).toBe('');
+      root.unmount(container);
+      expect(container.textContent).toBe('');
+    });
 
-      const container = document.createElement('div');
-      // Mount
-      ReactDOM.render(
-        <div>
-          <Foo />
-          <Bar />
-        </div>,
-        container,
-      );
-      // Root update
-      ReactDOM.render(
-        <div>
-          <Foo />
-          <Bar />
-        </div>,
-        container,
-      );
-      expect(ops).toEqual([
-        // Mount
-        'a: false',
-        'b: false',
-        // Root update
-        'a: false',
-        'b: false',
-        'Foo updated',
-        'Bar updated',
-        // Subsequent update (both a and b should have flushed)
-        'a: true',
-        'b: true',
-        'Foo updated',
-        'Bar updated',
-        // There should not be any additional updates
-      ]);
-    },
-  );
-
-  it('uses correct base state for setState inside render phase', () => {
-    const ops = [];
+    expect(container.textContent).toBe('');
+  });
 
+  it('uses correct base state for setState inside render phase', async () => {
     class Foo extends React.Component {
       state = {step: 0};
       render() {
         const memoizedStep = this.state.step;
         this.setState(baseState => {
           const baseStep = baseState.step;
-          ops.push(`base: ${baseStep}, memoized: ${memoizedStep}`);
+          Scheduler.log(`base: ${baseStep}, memoized: ${memoizedStep}`);
           return baseStep === 0 ? {step: 1} : null;
         });
         return null;
@@ -1264,48 +1283,54 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
-    expect(() => ReactDOM.render(<Foo />, container)).toErrorDev(
-      'Cannot update during an existing state transition',
-    );
-    expect(ops).toEqual(['base: 0, memoized: 0', 'base: 1, memoized: 1']);
+    const root = ReactDOMClient.createRoot(container);
+    await expect(async () => {
+      await act(() => {
+        root.render(<Foo />);
+      });
+    }).toErrorDev('Cannot update during an existing state transition');
+
+    assertLog(['base: 0, memoized: 0', 'base: 1, memoized: 1']);
   });
 
-  it('does not re-render if state update is null', () => {
+  it('does not re-render if state update is null', async () => {
     const container = document.createElement('div');
 
     let instance;
-    let ops = [];
     class Foo extends React.Component {
       render() {
         instance = this;
-        ops.push('render');
+        Scheduler.log('render');
         return <div />;
       }
     }
-    ReactDOM.render(<Foo />, container);
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Foo />);
+    });
 
-    ops = [];
-    instance.setState(() => null);
-    expect(ops).toEqual([]);
+    assertLog(['render']);
+    await act(() => {
+      instance.setState(() => null);
+    });
+    assertLog([]);
   });
 
-  // Will change once we switch to async by default
-  it('synchronously renders hidden subtrees', () => {
+  it('synchronously renders hidden subtrees', async () => {
     const container = document.createElement('div');
-    let ops = [];
 
     function Baz() {
-      ops.push('Baz');
+      Scheduler.log('Baz');
       return null;
     }
 
     function Bar() {
-      ops.push('Bar');
+      Scheduler.log('Bar');
       return null;
     }
 
     function Foo() {
-      ops.push('Foo');
+      Scheduler.log('Foo');
       return (
         <div>
           <div hidden={true}>
@@ -1316,14 +1341,18 @@ describe('ReactUpdates', () => {
       );
     }
 
-    // Mount
-    ReactDOM.render(<Foo />, container);
-    expect(ops).toEqual(['Foo', 'Bar', 'Baz']);
-    ops = [];
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      // Mount
+      root.render(<Foo />);
+    });
+    assertLog(['Foo', 'Bar', 'Baz']);
 
-    // Update
-    ReactDOM.render(<Foo />, container);
-    expect(ops).toEqual(['Foo', 'Bar', 'Baz']);
+    await act(() => {
+      // Update
+      root.render(<Foo />);
+    });
+    assertLog(['Foo', 'Bar', 'Baz']);
   });
 
   // @gate www
@@ -1383,18 +1412,33 @@ describe('ReactUpdates', () => {
     expect(hiddenDiv.innerHTML).toBe('<p>bar 1</p>');
   });
 
-  it('can render ridiculously large number of roots without triggering infinite update loop error', () => {
+  it('can render ridiculously large number of roots without triggering infinite update loop error', async () => {
+    function Component({trigger}) {
+      const [state, setState] = React.useState(0);
+
+      React.useEffect(() => {
+        if (trigger) {
+          Scheduler.log('Trigger');
+          setState(c => c + 1);
+        }
+      }, [trigger]);
+
+      return <div>{state}</div>;
+    }
+
     class Foo extends React.Component {
       componentDidMount() {
         const limit = 1200;
         for (let i = 0; i < limit; i++) {
           if (i < limit - 1) {
-            ReactDOM.render(<div />, document.createElement('div'));
+            ReactDOMClient.createRoot(document.createElement('div')).render(
+              <Component />,
+            );
           } else {
-            ReactDOM.render(<div />, document.createElement('div'), () => {
-              // The "nested update limit" error isn't thrown until setState
-              this.setState({});
-            });
+            // The "nested update limit" error isn't thrown until setState
+            ReactDOMClient.createRoot(document.createElement('div')).render(
+              <Component trigger={true} />,
+            );
           }
         }
       }
@@ -1403,11 +1447,16 @@ describe('ReactUpdates', () => {
       }
     }
 
-    const container = document.createElement('div');
-    ReactDOM.render(<Foo />, container);
+    const root = ReactDOMClient.createRoot(document.createElement('div'));
+    await act(() => {
+      root.render(<Foo />);
+    });
+
+    // Make sure the setState trigger runs.
+    assertLog(['Trigger']);
   });
 
-  it('resets the update counter for unrelated updates', () => {
+  it('resets the update counter for unrelated updates', async () => {
     const container = document.createElement('div');
     const ref = React.createRef();
 
@@ -1427,22 +1476,35 @@ describe('ReactUpdates', () => {
     }
 
     let limit = 55;
+    const root = ReactDOMClient.createRoot(container);
     expect(() => {
-      ReactDOM.render(<EventuallyTerminating ref={ref} />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<EventuallyTerminating ref={ref} />);
+      });
     }).toThrow('Maximum');
 
     // Verify that we don't go over the limit if these updates are unrelated.
     limit -= 10;
-    ReactDOM.render(<EventuallyTerminating ref={ref} />, container);
+    await act(() => {
+      root.render(<EventuallyTerminating ref={ref} />);
+    });
     expect(container.textContent).toBe(limit.toString());
-    ref.current.setState({step: 0});
+
+    await act(() => {
+      ref.current.setState({step: 0});
+    });
     expect(container.textContent).toBe(limit.toString());
-    ref.current.setState({step: 0});
+
+    await act(() => {
+      ref.current.setState({step: 0});
+    });
     expect(container.textContent).toBe(limit.toString());
 
     limit += 10;
     expect(() => {
-      ref.current.setState({step: 0});
+      ReactDOM.flushSync(() => {
+        ref.current.setState({step: 0});
+      });
     }).toThrow('Maximum');
     expect(ref.current).toBe(null);
   });
@@ -1450,12 +1512,15 @@ describe('ReactUpdates', () => {
   it('does not fall into an infinite update loop', () => {
     class NonTerminating extends React.Component {
       state = {step: 0};
+
       componentDidMount() {
         this.setState({step: 1});
       }
-      UNSAFE_componentWillUpdate() {
+
+      componentDidUpdate() {
         this.setState({step: 2});
       }
+
       render() {
         return (
           <div>
@@ -1467,8 +1532,12 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+
     expect(() => {
-      ReactDOM.render(<NonTerminating />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<NonTerminating />);
+      });
     }).toThrow('Maximum');
   });
 
@@ -1482,12 +1551,15 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
     expect(() => {
-      ReactDOM.render(<NonTerminating />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<NonTerminating />);
+      });
     }).toThrow('Maximum');
   });
 
-  it('can recover after falling into an infinite update loop', () => {
+  it('can recover after falling into an infinite update loop', async () => {
     class NonTerminating extends React.Component {
       state = {step: 0};
       componentDidMount() {
@@ -1512,27 +1584,36 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
     expect(() => {
-      ReactDOM.render(<NonTerminating />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<NonTerminating />);
+      });
     }).toThrow('Maximum');
 
-    ReactDOM.render(<Terminating />, container);
+    await act(() => {
+      root.render(<Terminating />);
+    });
     expect(container.textContent).toBe('1');
 
     expect(() => {
-      ReactDOM.render(<NonTerminating />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<NonTerminating />);
+      });
     }).toThrow('Maximum');
-
-    ReactDOM.render(<Terminating />, container);
+    await act(() => {
+      root.render(<Terminating />);
+    });
     expect(container.textContent).toBe('1');
   });
 
   it('does not fall into mutually recursive infinite update loop with same container', () => {
     // Note: this test would fail if there were two or more different roots.
-
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
     class A extends React.Component {
       componentDidMount() {
-        ReactDOM.render(<B />, container);
+        root.render(<B />);
       }
       render() {
         return null;
@@ -1541,16 +1622,17 @@ describe('ReactUpdates', () => {
 
     class B extends React.Component {
       componentDidMount() {
-        ReactDOM.render(<A />, container);
+        root.render(<A />);
       }
       render() {
         return null;
       }
     }
 
-    const container = document.createElement('div');
     expect(() => {
-      ReactDOM.render(<A />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<A />);
+      });
     }).toThrow('Maximum');
   });
 
@@ -1582,14 +1664,17 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
     expect(() => {
-      ReactDOM.render(<NonTerminating />, container);
+      ReactDOM.flushSync(() => {
+        root.render(<NonTerminating />);
+      });
     }).toThrow('Maximum');
   });
 
-  it('can schedule ridiculously many updates within the same batch without triggering a maximum update error', () => {
+  it('can schedule ridiculously many updates within the same batch without triggering a maximum update error', async () => {
     const subscribers = [];
-
+    const limit = 1200;
     class Child extends React.Component {
       state = {value: 'initial'};
       componentDidMount() {
@@ -1603,7 +1688,7 @@ describe('ReactUpdates', () => {
     class App extends React.Component {
       render() {
         const children = [];
-        for (let i = 0; i < 1200; i++) {
+        for (let i = 0; i < limit; i++) {
           children.push(<Child key={i} />);
         }
         return children;
@@ -1611,13 +1696,18 @@ describe('ReactUpdates', () => {
     }
 
     const container = document.createElement('div');
-    ReactDOM.render(<App />, container);
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<App />);
+    });
 
-    ReactDOM.unstable_batchedUpdates(() => {
+    await act(() => {
       subscribers.forEach(s => {
         s.setState({value: 'update'});
       });
     });
+
+    expect(subscribers.length).toBe(limit);
   });
 
   // TODO: Replace this branch with @gate pragmas
@@ -1673,8 +1763,9 @@ describe('ReactUpdates', () => {
       }
 
       const container = document.createElement('div');
+      const root = ReactDOMClient.createRoot(container);
       await act(() => {
-        ReactDOM.render(<Terminating />, container);
+        root.render(<Terminating />);
       });
       expect(container.textContent).toBe('50');
       await act(() => {
@@ -1696,8 +1787,9 @@ describe('ReactUpdates', () => {
       }
 
       const container = document.createElement('div');
+      const root = ReactDOMClient.createRoot(container);
       await act(() => {
-        ReactDOM.render(<Terminating />, container);
+        root.render(<Terminating />);
       });
 
       assertLog(['Done']);