[utils] Fix isPlainObject to work across realm (mui#39981)

packages/mui-utils/src/deepmerge.test.ts

@@ -1,4 +1,5 @@
 import { expect } from 'chai';
+import { runInNewContext } from 'vm';
 import deepmerge from './deepmerge';
 
 describe('deepmerge', () => {
@@ -11,6 +12,19 @@ describe('deepmerge', () => {
     expect({}).not.to.have.property('isAdmin');
   });
 
+  it('should merge objects across realms', function test() {
+    if (!/jsdom/.test(window.navigator.userAgent)) {
+      // vm is only available in Node.js.
+      // We could use https://github.com/browserify/vm-browserify to run the test in an iframe when
+      // in Karma but it doesn't seem we need to go as far.
+      this.skip();
+    }
+
+    const vmObject = runInNewContext('({hello: "realm"})');
+    const result = deepmerge({ hello: 'original' }, vmObject);
+    expect(result.hello).to.equal('realm');
+  });
+
   // https://github.com/mui/material-ui/issues/20095
   it('should not merge HTML elements', () => {
     const element = document.createElement('div');

packages/mui-utils/src/deepmerge.ts

@@ -1,5 +1,17 @@
+// https://github.com/sindresorhus/is-plain-obj/blob/main/index.js
 export function isPlainObject(item: unknown): item is Record<keyof any, unknown> {
-  return item !== null && typeof item === 'object' && item.constructor === Object;
+  if (typeof item !== 'object' || item === null) {
+    return false;
+  }
+
+  const prototype = Object.getPrototypeOf(item);
+  return (
+    (prototype === null ||
+      prototype === Object.prototype ||
+      Object.getPrototypeOf(prototype) === null) &&
+    !(Symbol.toStringTag in item) &&
+    !(Symbol.iterator in item)
+  );
 }
 
 export interface DeepmergeOptions {

test/karma.conf.js

@@ -194,6 +194,7 @@ module.exports = function setKarmaConfig(config) {
           stream: false,
           // required by enzyme > cheerio > parse5
           util: require.resolve('util/'),
+          vm: false,
         },
       },
       // TODO: 'browserslist:modern'