[Offscreen] Mount/unmount layout effects

Exposes the Offscreen component type and implements basic support for
mount/unmounting layout effects when the visibility is toggled.

Mostly it works the same way as hidden Suspense trees, which use the
same internal fiber type. I had to add an extra bailout, though, that
doesn't apply to the Suspense case but does apply to Offscreen
components: a hidden Offscreen tree will eventually render at low
priority, and when we it does, its `subtreeTag` will have effects
scheduled on it. So I added a check to the layout phase where, if the
subtree is hidden, we skip over the subtree entirely. An alternate
design would be to clear the subtree flags in the render phase, but I
prefer doing it this way since it's harder to mess up.

We also need an API to enable the same thing for passive effects. This
is not yet implemented.

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

@@ -2303,33 +2303,40 @@ function commitLayoutEffects_begin(
     const fiber = nextEffect;
     const firstChild = fiber.child;
 
-    if (enableSuspenseLayoutEffectSemantics && isModernRoot) {
+    if (
+      enableSuspenseLayoutEffectSemantics &&
+      fiber.tag === OffscreenComponent &&
+      isModernRoot
+    ) {
       // Keep track of the current Offscreen stack's state.
-      if (fiber.tag === OffscreenComponent) {
-        const current = fiber.alternate;
-        const wasHidden = current !== null && current.memoizedState !== null;
-        const isHidden = fiber.memoizedState !== null;
-
-        const newOffscreenSubtreeIsHidden =
-          isHidden || offscreenSubtreeIsHidden;
-        const newOffscreenSubtreeWasHidden =
-          wasHidden || offscreenSubtreeWasHidden;
-
-        if (
-          newOffscreenSubtreeIsHidden !== offscreenSubtreeIsHidden ||
-          newOffscreenSubtreeWasHidden !== offscreenSubtreeWasHidden
-        ) {
+      const isHidden = fiber.memoizedState !== null;
+      const newOffscreenSubtreeIsHidden = isHidden || offscreenSubtreeIsHidden;
+      if (newOffscreenSubtreeIsHidden) {
+        // The Offscreen tree is hidden. Skip over its layout effects.
+        commitLayoutMountEffects_complete(subtreeRoot, root, committedLanes);
+        continue;
+      } else {
+        if ((fiber.subtreeFlags & LayoutMask) !== NoFlags) {
+          const current = fiber.alternate;
+          const wasHidden = current !== null && current.memoizedState !== null;
+          const newOffscreenSubtreeWasHidden =
+            wasHidden || offscreenSubtreeWasHidden;
           const prevOffscreenSubtreeIsHidden = offscreenSubtreeIsHidden;
           const prevOffscreenSubtreeWasHidden = offscreenSubtreeWasHidden;
 
           // Traverse the Offscreen subtree with the current Offscreen as the root.
           offscreenSubtreeIsHidden = newOffscreenSubtreeIsHidden;
           offscreenSubtreeWasHidden = newOffscreenSubtreeWasHidden;
-          commitLayoutEffects_begin(
-            fiber, // New root; bubble back up to here and stop.
-            root,
-            committedLanes,
-          );
+          let child = firstChild;
+          while (child !== null) {
+            nextEffect = child;
+            commitLayoutEffects_begin(
+              child, // New root; bubble back up to here and stop.
+              root,
+              committedLanes,
+            );
+            child = child.sibling;
+          }
 
           // Restore Offscreen state and resume in our-progress traversal.
           nextEffect = fiber;

packages/react-reconciler/src/ReactFiberCommitWork.old.js

@@ -2303,33 +2303,40 @@ function commitLayoutEffects_begin(
     const fiber = nextEffect;
     const firstChild = fiber.child;
 
-    if (enableSuspenseLayoutEffectSemantics && isModernRoot) {
+    if (
+      enableSuspenseLayoutEffectSemantics &&
+      fiber.tag === OffscreenComponent &&
+      isModernRoot
+    ) {
       // Keep track of the current Offscreen stack's state.
-      if (fiber.tag === OffscreenComponent) {
-        const current = fiber.alternate;
-        const wasHidden = current !== null && current.memoizedState !== null;
-        const isHidden = fiber.memoizedState !== null;
-
-        const newOffscreenSubtreeIsHidden =
-          isHidden || offscreenSubtreeIsHidden;
-        const newOffscreenSubtreeWasHidden =
-          wasHidden || offscreenSubtreeWasHidden;
-
-        if (
-          newOffscreenSubtreeIsHidden !== offscreenSubtreeIsHidden ||
-          newOffscreenSubtreeWasHidden !== offscreenSubtreeWasHidden
-        ) {
+      const isHidden = fiber.memoizedState !== null;
+      const newOffscreenSubtreeIsHidden = isHidden || offscreenSubtreeIsHidden;
+      if (newOffscreenSubtreeIsHidden) {
+        // The Offscreen tree is hidden. Skip over its layout effects.
+        commitLayoutMountEffects_complete(subtreeRoot, root, committedLanes);
+        continue;
+      } else {
+        if ((fiber.subtreeFlags & LayoutMask) !== NoFlags) {
+          const current = fiber.alternate;
+          const wasHidden = current !== null && current.memoizedState !== null;
+          const newOffscreenSubtreeWasHidden =
+            wasHidden || offscreenSubtreeWasHidden;
           const prevOffscreenSubtreeIsHidden = offscreenSubtreeIsHidden;
           const prevOffscreenSubtreeWasHidden = offscreenSubtreeWasHidden;
 
           // Traverse the Offscreen subtree with the current Offscreen as the root.
           offscreenSubtreeIsHidden = newOffscreenSubtreeIsHidden;
           offscreenSubtreeWasHidden = newOffscreenSubtreeWasHidden;
-          commitLayoutEffects_begin(
-            fiber, // New root; bubble back up to here and stop.
-            root,
-            committedLanes,
-          );
+          let child = firstChild;
+          while (child !== null) {
+            nextEffect = child;
+            commitLayoutEffects_begin(
+              child, // New root; bubble back up to here and stop.
+              root,
+              committedLanes,
+            );
+            child = child.sibling;
+          }
 
           // Restore Offscreen state and resume in our-progress traversal.
           nextEffect = fiber;

packages/react-reconciler/src/__tests__/ReactOffscreen-test.js

@@ -2,7 +2,9 @@ let React;
 let ReactNoop;
 let Scheduler;
 let LegacyHidden;
+let Offscreen;
 let useState;
+let useLayoutEffect;
 
 describe('ReactOffscreen', () => {
   beforeEach(() => {
@@ -12,7 +14,9 @@ describe('ReactOffscreen', () => {
     ReactNoop = require('react-noop-renderer');
     Scheduler = require('scheduler');
     LegacyHidden = React.unstable_LegacyHidden;
+    Offscreen = React.unstable_Offscreen;
     useState = React.useState;
+    useLayoutEffect = React.useLayoutEffect;
   });
 
   function Text(props) {
@@ -169,4 +173,96 @@ describe('ReactOffscreen', () => {
       </>,
     );
   });
+
+  // @gate experimental
+  // @gate enableSuspenseLayoutEffectSemantics
+  it('mounts without layout effects', async () => {
+    function Child({text}) {
+      useLayoutEffect(() => {
+        Scheduler.unstable_yieldValue('Mount layout');
+        return () => {
+          Scheduler.unstable_yieldValue('Unmount layout');
+        };
+      }, []);
+      return <Text text="Child" />;
+    }
+
+    const root = ReactNoop.createRoot();
+
+    // Mount hidden tree.
+    await ReactNoop.act(async () => {
+      root.render(
+        <Offscreen mode="hidden">
+          <Child />
+        </Offscreen>,
+      );
+    });
+    // No layout effect.
+    expect(Scheduler).toHaveYielded(['Child']);
+    // TODO: Offscreen does not yet hide/unhide children correctly. Until we do,
+    // it should only be used inside a host component wrapper whose visibility
+    // is toggled simultaneously.
+    expect(root).toMatchRenderedOutput(<span prop="Child" />);
+
+    // Unhide the tree. The layout effect is mounted.
+    await ReactNoop.act(async () => {
+      root.render(
+        <Offscreen mode="visible">
+          <Child />
+        </Offscreen>,
+      );
+    });
+    expect(Scheduler).toHaveYielded(['Child', 'Mount layout']);
+    expect(root).toMatchRenderedOutput(<span prop="Child" />);
+  });
+
+  // @gate experimental
+  // @gate enableSuspenseLayoutEffectSemantics
+  it('mounts/unmounts layout effects when visibility changes', async () => {
+    function Child({text}) {
+      useLayoutEffect(() => {
+        Scheduler.unstable_yieldValue('Mount layout');
+        return () => {
+          Scheduler.unstable_yieldValue('Unmount layout');
+        };
+      }, []);
+      return <Text text="Child" />;
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(
+        <Offscreen mode="visible">
+          <Child />
+        </Offscreen>,
+      );
+    });
+    expect(Scheduler).toHaveYielded(['Child', 'Mount layout']);
+    expect(root).toMatchRenderedOutput(<span prop="Child" />);
+
+    // Hide the tree. The layout effect is unmounted.
+    await ReactNoop.act(async () => {
+      root.render(
+        <Offscreen mode="hidden">
+          <Child />
+        </Offscreen>,
+      );
+    });
+    expect(Scheduler).toHaveYielded(['Unmount layout', 'Child']);
+    // TODO: Offscreen does not yet hide/unhide children correctly. Until we do,
+    // it should only be used inside a host component wrapper whose visibility
+    // is toggled simultaneously.
+    expect(root).toMatchRenderedOutput(<span prop="Child" />);
+
+    // Unhide the tree. The layout effect is re-mounted.
+    await ReactNoop.act(async () => {
+      root.render(
+        <Offscreen mode="visible">
+          <Child />
+        </Offscreen>,
+      );
+    });
+    expect(Scheduler).toHaveYielded(['Child', 'Mount layout']);
+    expect(root).toMatchRenderedOutput(<span prop="Child" />);
+  });
 });

packages/react/index.classic.fb.js

@@ -35,6 +35,7 @@ export {
   StrictMode,
   Suspense,
   unstable_LegacyHidden,
+  unstable_Offscreen,
   createElement,
   cloneElement,
   isValidElement,

packages/react/index.experimental.js

@@ -33,6 +33,7 @@ export {
   StrictMode,
   Suspense,
   unstable_LegacyHidden,
+  unstable_Offscreen,
   createElement,
   cloneElement,
   isValidElement,

packages/react/index.js

@@ -74,6 +74,7 @@ export {
   SuspenseList,
   SuspenseList as unstable_SuspenseList,
   unstable_LegacyHidden,
+  unstable_Offscreen,
   unstable_Scope,
   unstable_useOpaqueIdentifier,
   unstable_getCacheForType,

packages/react/index.modern.fb.js

@@ -35,6 +35,7 @@ export {
   StrictMode,
   Suspense,
   unstable_LegacyHidden,
+  unstable_Offscreen,
   createElement,
   cloneElement,
   isValidElement,

packages/react/src/React.js

@@ -16,6 +16,7 @@ import {
   REACT_SUSPENSE_TYPE,
   REACT_SUSPENSE_LIST_TYPE,
   REACT_LEGACY_HIDDEN_TYPE,
+  REACT_OFFSCREEN_TYPE,
   REACT_SCOPE_TYPE,
   REACT_CACHE_TYPE,
 } from 'shared/ReactSymbols';
@@ -112,6 +113,7 @@ export {
   useDeferredValue,
   REACT_SUSPENSE_LIST_TYPE as SuspenseList,
   REACT_LEGACY_HIDDEN_TYPE as unstable_LegacyHidden,
+  REACT_OFFSCREEN_TYPE as unstable_Offscreen,
   getCacheForType as unstable_getCacheForType,
   useCacheRefresh as unstable_useCacheRefresh,
   REACT_CACHE_TYPE as unstable_Cache,

packages/shared/isValidElementType.js

@@ -21,6 +21,7 @@ import {
   REACT_LAZY_TYPE,
   REACT_SCOPE_TYPE,
   REACT_LEGACY_HIDDEN_TYPE,
+  REACT_OFFSCREEN_TYPE,
   REACT_CACHE_TYPE,
 } from 'shared/ReactSymbols';
 import {enableScopeAPI, enableCache} from './ReactFeatureFlags';
@@ -44,6 +45,7 @@ export default function isValidElementType(type: mixed) {
     type === REACT_SUSPENSE_TYPE ||
     type === REACT_SUSPENSE_LIST_TYPE ||
     type === REACT_LEGACY_HIDDEN_TYPE ||
+    type === REACT_OFFSCREEN_TYPE ||
     (enableScopeAPI && type === REACT_SCOPE_TYPE) ||
     (enableCache && type === REACT_CACHE_TYPE)
   ) {