[FORKED] Bugfix: Revealing a hidden update

This fixes a bug I discovered related to revealing a hidden Offscreen
tree. When this happens, we include in that render all the updates that
had previously been deferred — that is, all the updates that would have
already committed if the tree weren't hidden. This is necessary to avoid
tearing with the surrounding contents. (This was the "flickering"
Suspense bug we found a few years ago: facebook#18411.)

The way we do this is by tracking the lanes of the updates that were
deferred by a hidden tree. These are the "base" lanes. Then, in order
to reveal the hidden tree, we process any update that matches one of
those base lanes.

The bug I discovered is that some of these base lanes may include
updates that were not present at the time the tree was hidden. We cannot
flush those updates earlier that the surrounding contents — that, too,
could cause tearing.

The crux of the problem is that we sometimes reuse the same lane for
base updates and for non-base updates. So the lane alone isn't
sufficient to distinguish between these cases. We must track this in
some other way.

The solution I landed upon was to add an extra OffscreenLane bit to any
update that is made to a hidden tree. Then later when we reveal the
tree, we'll know not to treat them as base updates.

The extra OffscreenLane bit is removed as soon as that lane is committed
by the root (markRootFinished) — at that point, it gets
"upgraded" to a base update.

The trickiest part of this algorithm is reliably detecting when an
update is made to a hidden tree. What makes this challenging is when the
update is received during a concurrent event, while a render is already
in progress — it's possible the work-in-progress render is about to
flip the visibility of the tree that's being updated, leading to a race
condition.

To avoid a race condition, we will wait to read the visibility of the
tree until the current render has finished. In other words, this makes
it an atomic operation. Most of this logic was already implemented
in facebook#24663.

Because this bugfix depends on a moderately risky refactor to the update
queue (facebook#24663), it only works in the "new" reconciler fork. We will roll
it out gradually to www before landing in the main fork.

packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js

@@ -90,8 +90,10 @@ import type {Lanes, Lane} from './ReactFiberLane.new';
 import {
   NoLane,
   NoLanes,
+  OffscreenLane,
   isSubsetOfLanes,
   mergeLanes,
+  removeLanes,
   isTransitionLane,
   intersectLanes,
   markRootEntangled,
@@ -108,6 +110,7 @@ import {StrictLegacyMode} from './ReactTypeOfMode';
 import {
   markSkippedUpdateLanes,
   isUnsafeClassRenderPhaseUpdate,
+  getWorkInProgressRootRenderLanes,
 } from './ReactFiberWorkLoop.new';
 import {
   enqueueConcurrentClassUpdate,
@@ -523,9 +526,23 @@ export function processUpdateQueue<State>(
 
     let update = firstBaseUpdate;
     do {
-      const updateLane = update.lane;
+      // TODO: Don't need this field anymore
       const updateEventTime = update.eventTime;
-      if (!isSubsetOfLanes(renderLanes, updateLane)) {
+
+      // An extra OffscreenLane bit is added to updates that were made to
+      // a hidden tree, so that we can distinguish them from updates that were
+      // already there when the tree was hidden.
+      const updateLane = removeLanes(update.lane, OffscreenLane);
+      const isHiddenUpdate = updateLane !== update.lane;
+
+      // Check if this update was made while the tree was hidden. If so, then
+      // it's not a "base" update and we should disregard the extra base lanes
+      // that were added to renderLanes when we entered the Offscreen tree.
+      const shouldSkipUpdate = isHiddenUpdate
+        ? !isSubsetOfLanes(getWorkInProgressRootRenderLanes(), updateLane)
+        : !isSubsetOfLanes(renderLanes, updateLane);
+
+      if (shouldSkipUpdate) {
         // Priority is insufficient. Skip this update. If this is the first
         // skipped update, the previous update/state is the new base
         // update/state.

packages/react-reconciler/src/ReactFiberConcurrentUpdates.new.js

@@ -17,14 +17,21 @@ import type {
   Update as ClassUpdate,
 } from './ReactFiberClassUpdateQueue.new';
 import type {Lane, Lanes} from './ReactFiberLane.new';
+import type {OffscreenInstance} from './ReactFiberOffscreenComponent';
 
 import {warnAboutUpdateOnNotYetMountedFiberInDEV} from './ReactFiberWorkLoop.new';
-import {NoLane, NoLanes, mergeLanes} from './ReactFiberLane.new';
+import {
+  NoLane,
+  NoLanes,
+  mergeLanes,
+  markHiddenUpdate,
+} from './ReactFiberLane.new';
 import {NoFlags, Placement, Hydrating} from './ReactFiberFlags';
-import {HostRoot} from './ReactWorkTags';
+import {HostRoot, OffscreenComponent} from './ReactWorkTags';
 
-type ConcurrentUpdate = {
+export type ConcurrentUpdate = {
   next: ConcurrentUpdate,
+  lane: Lane,
 };
 
 type ConcurrentQueue = {
@@ -38,11 +45,13 @@ type ConcurrentQueue = {
 const concurrentQueues: Array<any> = [];
 let concurrentQueuesIndex = 0;
 
-export function finishQueueingConcurrentUpdates(): Lanes {
+let concurrentlyUpdatedLanes: Lanes = NoLanes;
+
+export function finishQueueingConcurrentUpdates(): void {
   const endIndex = concurrentQueuesIndex;
   concurrentQueuesIndex = 0;
 
-  let lanes = NoLanes;
+  concurrentlyUpdatedLanes = NoLanes;
 
   let i = 0;
   while (i < endIndex) {
@@ -68,12 +77,13 @@ export function finishQueueingConcurrentUpdates(): Lanes {
     }
 
     if (lane !== NoLane) {
-      lanes = mergeLanes(lanes, lane);
-      markUpdateLaneFromFiberToRoot(fiber, lane);
+      markUpdateLaneFromFiberToRoot(fiber, update, lane);
     }
   }
+}
 
-  return lanes;
+export function getConcurrentlyUpdatedLanes(): Lanes {
+  return concurrentlyUpdatedLanes;
 }
 
 function enqueueUpdate(
@@ -89,6 +99,8 @@ function enqueueUpdate(
   concurrentQueues[concurrentQueuesIndex++] = update;
   concurrentQueues[concurrentQueuesIndex++] = lane;
 
+  concurrentlyUpdatedLanes = mergeLanes(concurrentlyUpdatedLanes, lane);
+
   // The fiber's `lane` field is used in some places to check if any work is
   // scheduled, to perform an eager bailout, so we need to update it immediately.
   // TODO: We should probably move this to the "shared" queue instead.
@@ -151,27 +163,47 @@ export function unsafe_markUpdateLaneFromFiberToRoot(
   sourceFiber: Fiber,
   lane: Lane,
 ): FiberRoot | null {
-  markUpdateLaneFromFiberToRoot(sourceFiber, lane);
+  markUpdateLaneFromFiberToRoot(sourceFiber, null, lane);
   return getRootForUpdatedFiber(sourceFiber);
 }
 
-function markUpdateLaneFromFiberToRoot(sourceFiber: Fiber, lane: Lane): void {
+function markUpdateLaneFromFiberToRoot(
+  sourceFiber: Fiber,
+  update: ConcurrentUpdate | null,
+  lane: Lane,
+): void {
   // Update the source fiber's lanes
   sourceFiber.lanes = mergeLanes(sourceFiber.lanes, lane);
   let alternate = sourceFiber.alternate;
   if (alternate !== null) {
     alternate.lanes = mergeLanes(alternate.lanes, lane);
   }
   // Walk the parent path to the root and update the child lanes.
+  let isHidden = false;
   let parent = sourceFiber.return;
+  let node = sourceFiber;
   while (parent !== null) {
     parent.childLanes = mergeLanes(parent.childLanes, lane);
     alternate = parent.alternate;
     if (alternate !== null) {
       alternate.childLanes = mergeLanes(alternate.childLanes, lane);
     }
+
+    if (parent.tag === OffscreenComponent) {
+      const offscreenInstance: OffscreenInstance = parent.stateNode;
+      if (offscreenInstance.isHidden) {
+        isHidden = true;
+      }
+    }
+
+    node = parent;
     parent = parent.return;
   }
+
+  if (isHidden && update !== null && node.tag === HostRoot) {
+    const root: FiberRoot = node.stateNode;
+    markHiddenUpdate(root, update, lane);
+  }
 }
 
 function getRootForUpdatedFiber(sourceFiber: Fiber): FiberRoot | null {

packages/react-reconciler/src/ReactFiberHooks.new.js

@@ -44,6 +44,7 @@ import {
 import {
   NoLane,
   SyncLane,
+  OffscreenLane,
   NoLanes,
   isSubsetOfLanes,
   includesBlockingLane,
@@ -83,6 +84,7 @@ import {
 } from './ReactHookEffectTags';
 import {
   getWorkInProgressRoot,
+  getWorkInProgressRootRenderLanes,
   scheduleUpdateOnFiber,
   requestUpdateLane,
   requestEventTime,
@@ -811,8 +813,20 @@ function updateReducer<S, I, A>(
     let newBaseQueueLast = null;
     let update = first;
     do {
-      const updateLane = update.lane;
-      if (!isSubsetOfLanes(renderLanes, updateLane)) {
+      // An extra OffscreenLane bit is added to updates that were made to
+      // a hidden tree, so that we can distinguish them from updates that were
+      // already there when the tree was hidden.
+      const updateLane = removeLanes(update.lane, OffscreenLane);
+      const isHiddenUpdate = updateLane !== update.lane;
+
+      // Check if this update was made while the tree was hidden. If so, then
+      // it's not a "base" update and we should disregard the extra base lanes
+      // that were added to renderLanes when we entered the Offscreen tree.
+      const shouldSkipUpdate = isHiddenUpdate
+        ? !isSubsetOfLanes(getWorkInProgressRootRenderLanes(), updateLane)
+        : !isSubsetOfLanes(renderLanes, updateLane);
+
+      if (shouldSkipUpdate) {
         // Priority is insufficient. Skip this update. If this is the first
         // skipped update, the previous update/state is the new base
         // update/state.

packages/react-reconciler/src/ReactFiberLane.new.js

@@ -9,6 +9,7 @@
 
 import type {FiberRoot} from './ReactInternalTypes';
 import type {Transition} from './ReactFiberTracingMarkerComponent.new';
+import type {ConcurrentUpdate} from './ReactFiberConcurrentUpdates.new';
 
 // TODO: Ideally these types would be opaque but that doesn't work well with
 // our reconciler fork infra, since these leak into non-reconciler packages.
@@ -648,6 +649,7 @@ export function markRootFinished(root: FiberRoot, remainingLanes: Lanes) {
   const entanglements = root.entanglements;
   const eventTimes = root.eventTimes;
   const expirationTimes = root.expirationTimes;
+  const hiddenUpdates = root.hiddenUpdates;
 
   // Clear the lanes that no longer have pending work
   let lanes = noLongerPendingLanes;
@@ -659,6 +661,21 @@ export function markRootFinished(root: FiberRoot, remainingLanes: Lanes) {
     eventTimes[index] = NoTimestamp;
     expirationTimes[index] = NoTimestamp;
 
+    const hiddenUpdatesForLane = hiddenUpdates[index];
+    if (hiddenUpdatesForLane !== null) {
+      hiddenUpdates[index] = null;
+      // "Hidden" updates are updates that were made to a hidden component. They
+      // have special logic associated with them because they may be entangled
+      // with updates that occur outside that tree. But once the outer tree
+      // commits, they behave like regular updates.
+      for (let i = 0; i < hiddenUpdatesForLane.length; i++) {
+        const update = hiddenUpdatesForLane[i];
+        if (update !== null) {
+          update.lane &= ~OffscreenLane;
+        }
+      }
+    }
+
     lanes &= ~lane;
   }
 }
@@ -694,6 +711,22 @@ export function markRootEntangled(root: FiberRoot, entangledLanes: Lanes) {
   }
 }
 
+export function markHiddenUpdate(
+  root: FiberRoot,
+  update: ConcurrentUpdate,
+  lane: Lane,
+) {
+  const index = laneToIndex(lane);
+  const hiddenUpdates = root.hiddenUpdates;
+  const hiddenUpdatesForLane = hiddenUpdates[index];
+  if (hiddenUpdatesForLane === null) {
+    hiddenUpdates[index] = [update];
+  } else {
+    hiddenUpdatesForLane.push(update);
+  }
+  update.lane = lane | OffscreenLane;
+}
+
 export function getBumpedLaneForHydration(
   root: FiberRoot,
   renderLanes: Lanes,

packages/react-reconciler/src/ReactFiberRoot.new.js

@@ -80,6 +80,8 @@ function FiberRootNode(
   this.entangledLanes = NoLanes;
   this.entanglements = createLaneMap(NoLanes);
 
+  this.hiddenUpdates = createLaneMap(null);
+
   this.identifierPrefix = identifierPrefix;
   this.onRecoverableError = onRecoverableError;
 

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -199,6 +199,7 @@ import {
 import {
   enqueueConcurrentRenderForLane,
   finishQueueingConcurrentUpdates,
+  getConcurrentlyUpdatedLanes,
 } from './ReactFiberConcurrentUpdates.new';
 
 import {
@@ -425,6 +426,10 @@ export function getWorkInProgressRoot(): FiberRoot | null {
   return workInProgressRoot;
 }
 
+export function getWorkInProgressRootRenderLanes(): Lanes {
+  return workInProgressRootRenderLanes;
+}
+
 export function requestEventTime() {
   if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {
     // We're inside React, so it's fine to read the actual time.
@@ -2059,7 +2064,7 @@ function commitRootImpl(
 
   // Make sure to account for lanes that were updated by a concurrent event
   // during the render phase; don't mark them as finished.
-  const concurrentlyUpdatedLanes = finishQueueingConcurrentUpdates();
+  const concurrentlyUpdatedLanes = getConcurrentlyUpdatedLanes();
   remainingLanes = mergeLanes(remainingLanes, concurrentlyUpdatedLanes);
 
   markRootFinished(root, remainingLanes);

packages/react-reconciler/src/ReactInternalTypes.js

@@ -27,6 +27,7 @@ import type {RootTag} from './ReactRootTags';
 import type {TimeoutHandle, NoTimeout} from './ReactFiberHostConfig';
 import type {Cache} from './ReactFiberCacheComponent.old';
 import type {Transition} from './ReactFiberTracingMarkerComponent.new';
+import type {ConcurrentUpdate} from './ReactFiberConcurrentUpdates.new';
 
 // Unwind Circular: moved from ReactFiberHooks.old
 export type HookType =
@@ -225,6 +226,7 @@ type BaseFiberRootProperties = {|
   callbackPriority: Lane,
   eventTimes: LaneMap<number>,
   expirationTimes: LaneMap<number>,
+  hiddenUpdates: LaneMap<Array<ConcurrentUpdate> | null>,
 
   pendingLanes: Lanes,
   suspendedLanes: Lanes,