Continuous updates should interrupt transitions

Even when updates are sync by default.

Discovered this quirk while working on facebook#21322. Previously, when sync
default updates are enabled, continuous updates are treated like
default updates. We implemented this by assigning DefaultLane to
continous updates. However, an unintended consequence of that approach
is that continuous updates would no longer interrupt transitions,
because default updates are not supposed to interrupt transitions.

To fix this, I changed the implementation to always assign separate
lanes for default and continuous updates. Then I entangle the
lanes together.

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

@@ -39,6 +39,7 @@ import {
   enableCache,
   enableSchedulingProfiler,
   enableUpdaterTracking,
+  enableSyncDefaultUpdates,
 } from 'shared/ReactFeatureFlags';
 import {isDevToolsPresent} from './ReactFiberDevToolsHook.new';
 
@@ -273,6 +274,18 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
     }
   }
 
+  if (
+    // TODO: Check for root override, once that lands
+    enableSyncDefaultUpdates &&
+    (nextLanes & InputContinuousLane) !== NoLanes
+  ) {
+    // When updates are sync by default, we entangle continous priority updates
+    // and default updates, so they render in the same batch. The only reason
+    // they use separate lanes is because continuous updates should interrupt
+    // transitions, but default updates should not.
+    nextLanes |= pendingLanes & DefaultLane;
+  }
+
   // Check for entangled lanes and add them to the batch.
   //
   // A lane is said to be entangled with another when it's not allowed to render

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

@@ -39,6 +39,7 @@ import {
   enableCache,
   enableSchedulingProfiler,
   enableUpdaterTracking,
+  enableSyncDefaultUpdates,
 } from 'shared/ReactFeatureFlags';
 import {isDevToolsPresent} from './ReactFiberDevToolsHook.old';
 
@@ -273,6 +274,18 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
     }
   }
 
+  if (
+    // TODO: Check for root override, once that lands
+    enableSyncDefaultUpdates &&
+    (nextLanes & InputContinuousLane) !== NoLanes
+  ) {
+    // When updates are sync by default, we entangle continous priority updates
+    // and default updates, so they render in the same batch. The only reason
+    // they use separate lanes is because continuous updates should interrupt
+    // transitions, but default updates should not.
+    nextLanes |= pendingLanes & DefaultLane;
+  }
+
   // Check for entangled lanes and add them to the batch.
   //
   // A lane is said to be entangled with another when it's not allowed to render

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

@@ -139,8 +139,8 @@ import {
   NoLanes,
   NoLane,
   SyncLane,
-  DefaultLane,
   DefaultHydrationLane,
+  DefaultLane,
   InputContinuousLane,
   InputContinuousHydrationLane,
   NoTimestamp,
@@ -437,13 +437,6 @@ export function requestUpdateLane(fiber: Fiber): Lane {
   // TODO: Move this type conversion to the event priority module.
   const updateLane: Lane = (getCurrentUpdatePriority(): any);
   if (updateLane !== NoLane) {
-    if (
-      enableSyncDefaultUpdates &&
-      (updateLane === InputContinuousLane ||
-        updateLane === InputContinuousHydrationLane)
-    ) {
-      return DefaultLane;
-    }
     return updateLane;
   }
 
@@ -454,13 +447,6 @@ export function requestUpdateLane(fiber: Fiber): Lane {
   // use that directly.
   // TODO: Move this type conversion to the event priority module.
   const eventLane: Lane = (getCurrentEventPriority(): any);
-  if (
-    enableSyncDefaultUpdates &&
-    (eventLane === InputContinuousLane ||
-      eventLane === InputContinuousHydrationLane)
-  ) {
-    return DefaultLane;
-  }
   return eventLane;
 }
 
@@ -814,7 +800,9 @@ function performConcurrentWorkOnRoot(root, didTimeout) {
   let exitStatus =
     enableSyncDefaultUpdates &&
     (includesSomeLane(lanes, DefaultLane) ||
-      includesSomeLane(lanes, DefaultHydrationLane))
+      includesSomeLane(lanes, InputContinuousLane) ||
+      includesSomeLane(lanes, DefaultHydrationLane) ||
+      includesSomeLane(lanes, InputContinuousHydrationLane))
       ? // Time slicing is disabled for default updates in this root.
         renderRootSync(root, lanes)
       : renderRootConcurrent(root, lanes);

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

@@ -139,8 +139,8 @@ import {
   NoLanes,
   NoLane,
   SyncLane,
-  DefaultLane,
   DefaultHydrationLane,
+  DefaultLane,
   InputContinuousLane,
   InputContinuousHydrationLane,
   NoTimestamp,
@@ -437,13 +437,6 @@ export function requestUpdateLane(fiber: Fiber): Lane {
   // TODO: Move this type conversion to the event priority module.
   const updateLane: Lane = (getCurrentUpdatePriority(): any);
   if (updateLane !== NoLane) {
-    if (
-      enableSyncDefaultUpdates &&
-      (updateLane === InputContinuousLane ||
-        updateLane === InputContinuousHydrationLane)
-    ) {
-      return DefaultLane;
-    }
     return updateLane;
   }
 
@@ -454,13 +447,6 @@ export function requestUpdateLane(fiber: Fiber): Lane {
   // use that directly.
   // TODO: Move this type conversion to the event priority module.
   const eventLane: Lane = (getCurrentEventPriority(): any);
-  if (
-    enableSyncDefaultUpdates &&
-    (eventLane === InputContinuousLane ||
-      eventLane === InputContinuousHydrationLane)
-  ) {
-    return DefaultLane;
-  }
   return eventLane;
 }
 
@@ -814,7 +800,9 @@ function performConcurrentWorkOnRoot(root, didTimeout) {
   let exitStatus =
     enableSyncDefaultUpdates &&
     (includesSomeLane(lanes, DefaultLane) ||
-      includesSomeLane(lanes, DefaultHydrationLane))
+      includesSomeLane(lanes, InputContinuousLane) ||
+      includesSomeLane(lanes, DefaultHydrationLane) ||
+      includesSomeLane(lanes, InputContinuousHydrationLane))
       ? // Time slicing is disabled for default updates in this root.
         renderRootSync(root, lanes)
       : renderRootConcurrent(root, lanes);

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

@@ -1405,18 +1405,6 @@ describe('ReactHooksWithNoopRenderer', () => {
         ReactNoop.unstable_runWithPriority(ContinuousEventPriority, () => {
           setParentState(false);
         });
-        if (gate(flags => flags.enableSyncDefaultUpdates)) {
-          // TODO: Default updates do not interrupt transition updates, to
-          // prevent starvation. However, when sync default updates are enabled,
-          // continuous updates are treated like default updates. In this case,
-          // we probably don't want this behavior; continuous should be allowed
-          // to interrupt.
-          expect(Scheduler).toFlushUntilNextPaint([
-            'Child two render',
-            'Child one commit',
-            'Child two commit',
-          ]);
-        }
         expect(Scheduler).toFlushUntilNextPaint([
           'Parent false render',
           'Parent false commit',

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

@@ -1,6 +1,8 @@
 let React;
 let ReactNoop;
 let Scheduler;
+let ContinuousEventPriority;
+let startTransition;
 let useState;
 let useEffect;
 
@@ -11,6 +13,9 @@ describe('ReactUpdatePriority', () => {
     React = require('react');
     ReactNoop = require('react-noop-renderer');
     Scheduler = require('scheduler');
+    ContinuousEventPriority = require('react-reconciler/constants')
+      .ContinuousEventPriority;
+    startTransition = React.unstable_startTransition;
     useState = React.useState;
     useEffect = React.useEffect;
   });
@@ -78,4 +83,52 @@ describe('ReactUpdatePriority', () => {
     // Now the idle update has flushed
     expect(Scheduler).toHaveYielded(['Idle: 2, Default: 2']);
   });
+
+  test('continuous updates should interrupt transisions', async () => {
+    const root = ReactNoop.createRoot();
+
+    let setCounter;
+    let setIsHidden;
+    function App() {
+      const [counter, _setCounter] = useState(1);
+      const [isHidden, _setIsHidden] = useState(false);
+      setCounter = _setCounter;
+      setIsHidden = _setIsHidden;
+      if (isHidden) {
+        return <Text text={'(hidden)'} />;
+      }
+      return (
+        <>
+          <Text text={'A' + counter} />
+          <Text text={'B' + counter} />
+          <Text text={'C' + counter} />
+        </>
+      );
+    }
+
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+    });
+    expect(Scheduler).toHaveYielded(['A1', 'B1', 'C1']);
+    expect(root).toMatchRenderedOutput('A1B1C1');
+
+    await ReactNoop.act(async () => {
+      startTransition(() => {
+        setCounter(2);
+      });
+      expect(Scheduler).toFlushAndYieldThrough(['A2']);
+      ReactNoop.unstable_runWithPriority(ContinuousEventPriority, () => {
+        setIsHidden(true);
+      });
+    });
+    expect(Scheduler).toHaveYielded([
+      // Because the hide update has continous priority, it should interrupt the
+      // in-progress transition
+      '(hidden)',
+      // When the transition resumes, it's a no-op because the children are
+      // now hidden.
+      '(hidden)',
+    ]);
+    expect(root).toMatchRenderedOutput('(hidden)');
+  });
 });

packages/react-reconciler/src/__tests__/SchedulingProfilerLabels-test.internal.js

@@ -168,18 +168,10 @@ describe('SchedulingProfiler labels', () => {
       event.initEvent('mouseover', true, true);
       dispatchAndSetCurrentEvent(targetRef.current, event);
     });
-    if (gate(flags => flags.enableSyncDefaultUpdates)) {
-      expect(clearedMarks).toContain(
-        `--schedule-state-update-${formatLanes(
-          ReactFiberLane.DefaultLane,
-        )}-App`,
-      );
-    } else {
-      expect(clearedMarks).toContain(
-        `--schedule-state-update-${formatLanes(
-          ReactFiberLane.InputContinuousLane,
-        )}-App`,
-      );
-    }
+    expect(clearedMarks).toContain(
+      `--schedule-state-update-${formatLanes(
+        ReactFiberLane.InputContinuousLane,
+      )}-App`,
+    );
   });
 });