Remove JND delay for non-transition updates (#26597)

Updates that are marked as part of a transition are allowed to block a
render from committing. Generally, other updates cannot — however,
there's one exception that's leftover from a previous iteration of our
Suspense architecture. If an update is not the result of a known urgent
event type — known as "Default" updates — then we allow it to suspend
briefly, as long as the delay is short enough that the user won't
notice. We refer to this delay as a "Just Noticable Difference" (JND)
delay. To illustrate, if the user has already waited 400ms for an update
to be reflected on the screen, the theory is that they won't notice if
you wait an additional 100ms. So React can suspend for a bit longer in
case more data comes in. The longer the user has already waited, the
longer the JND.

While we still believe this theory is sound from a UX perspective, we no
longer think the implementation complexity is worth it. The main thing
that's changed is how we handle Default updates. We used to render
Default updates concurrently (i.e. they were time sliced, and were
scheduled with postTask), but now they are blocking. Soon, they will
also be scheduled with rAF, too, which means by the end of the next rAF,
they will have either finished rendering or the main thread will be
blocked until they do. There are various motivations for this but part
of the rationale is that anything that can be made non-blocking should
be marked as a Transition, anyway, so it's not worth adding
implementation complexity to Default.

This commit removes the JND delay for Default updates. They will now
commit immediately once the render phase is complete, even if a
component suspends.

Author: acdlite

packages/react-cache/src/__tests__/ReactCacheOld-test.internal.js

@@ -236,7 +236,7 @@ describe('ReactCache', () => {
 
     jest.advanceTimersByTime(100);
     assertLog(['Promise resolved [4]']);
-    await waitForAll([1, 4, 'Suspend! [5]', 'Loading...']);
+    await waitForAll([1, 4, 'Suspend! [5]']);
 
     jest.advanceTimersByTime(100);
     assertLog(['Promise resolved [5]']);
@@ -264,7 +264,7 @@ describe('ReactCache', () => {
     ]);
     jest.advanceTimersByTime(100);
     assertLog(['Promise resolved [2]']);
-    await waitForAll([1, 2, 'Suspend! [3]', 'Loading...']);
+    await waitForAll([1, 2, 'Suspend! [3]']);
 
     jest.advanceTimersByTime(100);
     assertLog(['Promise resolved [3]']);

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -145,7 +145,6 @@ import {
   includesExpiredLane,
   getNextLanes,
   getLanesToRetrySynchronouslyOnError,
-  getMostRecentEventTime,
   markRootUpdated,
   markRootSuspended as markRootSuspended_dontCallThisOneDirectly,
   markRootPinged,
@@ -284,8 +283,6 @@ import {
 } from './ReactFiberRootScheduler';
 import {getMaskedContext, getUnmaskedContext} from './ReactFiberContext';
 
-const ceil = Math.ceil;
-
 const PossiblyWeakMap = typeof WeakMap === 'function' ? WeakMap : Map;
 
 const {
@@ -1193,38 +1190,6 @@ function finishConcurrentRender(
         break;
       }
 
-      if (!shouldForceFlushFallbacksInDEV()) {
-        // This is not a transition, but we did trigger an avoided state.
-        // Schedule a placeholder to display after a short delay, using the Just
-        // Noticeable Difference.
-        // TODO: Is the JND optimization worth the added complexity? If this is
-        // the only reason we track the event time, then probably not.
-        // Consider removing.
-
-        const mostRecentEventTime = getMostRecentEventTime(root, lanes);
-        const eventTimeMs = mostRecentEventTime;
-        const timeElapsedMs = now() - eventTimeMs;
-        const msUntilTimeout = jnd(timeElapsedMs) - timeElapsedMs;
-
-        // Don't bother with a very short suspense time.
-        if (msUntilTimeout > 10) {
-          // Instead of committing the fallback immediately, wait for more data
-          // to arrive.
-          root.timeoutHandle = scheduleTimeout(
-            commitRootWhenReady.bind(
-              null,
-              root,
-              finishedWork,
-              workInProgressRootRecoverableErrors,
-              workInProgressTransitions,
-              lanes,
-            ),
-            msUntilTimeout,
-          );
-          break;
-        }
-      }
-
       // Commit the placeholder.
       commitRootWhenReady(
         root,
@@ -3580,31 +3545,6 @@ export function resolveRetryWakeable(boundaryFiber: Fiber, wakeable: Wakeable) {
   retryTimedOutBoundary(boundaryFiber, retryLane);
 }
 
-// Computes the next Just Noticeable Difference (JND) boundary.
-// The theory is that a person can't tell the difference between small differences in time.
-// Therefore, if we wait a bit longer than necessary that won't translate to a noticeable
-// difference in the experience. However, waiting for longer might mean that we can avoid
-// showing an intermediate loading state. The longer we have already waited, the harder it
-// is to tell small differences in time. Therefore, the longer we've already waited,
-// the longer we can wait additionally. At some point we have to give up though.
-// We pick a train model where the next boundary commits at a consistent schedule.
-// These particular numbers are vague estimates. We expect to adjust them based on research.
-function jnd(timeElapsed: number) {
-  return timeElapsed < 120
-    ? 120
-    : timeElapsed < 480
-    ? 480
-    : timeElapsed < 1080
-    ? 1080
-    : timeElapsed < 1920
-    ? 1920
-    : timeElapsed < 3000
-    ? 3000
-    : timeElapsed < 4320
-    ? 4320
-    : ceil(timeElapsed / 1960) * 1960;
-}
-
 export function throwIfInfiniteUpdateLoopDetected() {
   if (nestedUpdateCount > NESTED_UPDATE_LIMIT) {
     nestedUpdateCount = 0;

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

@@ -732,13 +732,9 @@ describe('ReactExpiration', () => {
     expect(root).toMatchRenderedOutput('A0BC');
 
     await act(async () => {
-      if (gate(flags => flags.enableSyncDefaultUpdates)) {
-        React.startTransition(() => {
-          root.render(<App step={1} />);
-        });
-      } else {
+      React.startTransition(() => {
         root.render(<App step={1} />);
-      }
+      });
       await waitForAll(['Suspend! [A1]', 'Loading...']);
 
       // Lots of time elapses before the promise resolves

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

@@ -692,24 +692,16 @@ describe('ReactHooksWithNoopRenderer', () => {
       await waitForAll([0]);
       expect(root).toMatchRenderedOutput(<span prop={0} />);
 
-      if (gate(flags => flags.enableSyncDefaultUpdates)) {
-        React.startTransition(() => {
-          root.render(<Foo signal={false} />);
-        });
-      } else {
+      React.startTransition(() => {
         root.render(<Foo signal={false} />);
-      }
+      });
       await waitForAll(['Suspend!']);
       expect(root).toMatchRenderedOutput(<span prop={0} />);
 
       // Rendering again should suspend again.
-      if (gate(flags => flags.enableSyncDefaultUpdates)) {
-        React.startTransition(() => {
-          root.render(<Foo signal={false} />);
-        });
-      } else {
+      React.startTransition(() => {
         root.render(<Foo signal={false} />);
-      }
+      });
       await waitForAll(['Suspend!']);
     });
 
@@ -755,38 +747,25 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(root).toMatchRenderedOutput(<span prop="A:0" />);
 
       await act(async () => {
-        if (gate(flags => flags.enableSyncDefaultUpdates)) {
-          React.startTransition(() => {
-            root.render(<Foo signal={false} />);
-            setLabel('B');
-          });
-        } else {
+        React.startTransition(() => {
           root.render(<Foo signal={false} />);
           setLabel('B');
-        }
+        });
 
         await waitForAll(['Suspend!']);
         expect(root).toMatchRenderedOutput(<span prop="A:0" />);
 
         // Rendering again should suspend again.
-        if (gate(flags => flags.enableSyncDefaultUpdates)) {
-          React.startTransition(() => {
-            root.render(<Foo signal={false} />);
-          });
-        } else {
+        React.startTransition(() => {
           root.render(<Foo signal={false} />);
-        }
+        });
         await waitForAll(['Suspend!']);
 
         // Flip the signal back to "cancel" the update. However, the update to
         // label should still proceed. It shouldn't have been dropped.
-        if (gate(flags => flags.enableSyncDefaultUpdates)) {
-          React.startTransition(() => {
-            root.render(<Foo signal={true} />);
-          });
-        } else {
+        React.startTransition(() => {
           root.render(<Foo signal={true} />);
-        }
+        });
         await waitForAll(['B:0']);
         expect(root).toMatchRenderedOutput(<span prop="B:0" />);
       });

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

@@ -1414,10 +1414,12 @@ describe('ReactLazy', () => {
 
     // Swap the position of A and B
     root.update(<Parent swap={true} />);
-    await waitForAll(['Init B2', 'Loading...']);
-    jest.runAllTimers();
-
-    assertLog(['Did unmount: A', 'Did unmount: B']);
+    await waitForAll([
+      'Init B2',
+      'Loading...',
+      'Did unmount: A',
+      'Did unmount: B',
+    ]);
 
     // The suspense boundary should've triggered now.
     expect(root).toMatchRenderedOutput('Loading...');
@@ -1559,13 +1561,9 @@ describe('ReactLazy', () => {
     expect(root).toMatchRenderedOutput('AB');
 
     // Swap the position of A and B
-    if (gate(flags => flags.enableSyncDefaultUpdates)) {
-      React.startTransition(() => {
-        root.update(<Parent swap={true} />);
-      });
-    } else {
+    React.startTransition(() => {
       root.update(<Parent swap={true} />);
-    }
+    });
     await waitForAll(['Init B2', 'Loading...']);
     await resolveFakeImport(ChildB2);
     // We need to flush to trigger the second one to load.

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

@@ -9,6 +9,7 @@ let useState;
 let useEffect;
 let startTransition;
 let textCache;
+let waitFor;
 let waitForPaint;
 let assertLog;
 
@@ -28,6 +29,7 @@ describe('ReactOffscreen', () => {
     startTransition = React.startTransition;
 
     const InternalTestUtils = require('internal-test-utils');
+    waitFor = InternalTestUtils.waitFor;
     waitForPaint = InternalTestUtils.waitForPaint;
     assertLog = InternalTestUtils.assertLog;
 
@@ -407,7 +409,6 @@ describe('ReactOffscreen', () => {
     expect(root).toMatchRenderedOutput(<span hidden={true}>B1</span>);
   });
 
-  // Only works in new reconciler
   // @gate enableOffscreen
   test('detect updates to a hidden tree during a concurrent event', async () => {
     // This is a pretty complex test case. It relates to how we detect if an
@@ -442,17 +443,17 @@ describe('ReactOffscreen', () => {
       setOuter = _setOuter;
       return (
         <>
-          <span>
-            <Text text={'Outer: ' + outer} />
-          </span>
           <Offscreen mode={show ? 'visible' : 'hidden'}>
             <span>
               <Child outer={outer} />
             </span>
           </Offscreen>
+          <span>
+            <Text text={'Outer: ' + outer} />
+          </span>
           <Suspense fallback={<Text text="Loading..." />}>
             <span>
-              <AsyncText text={'Async: ' + outer} />
+              <Text text={'Sibling: ' + outer} />
             </span>
           </Suspense>
         </>
@@ -466,50 +467,41 @@ describe('ReactOffscreen', () => {
       root.render(<App show={true} />);
     });
     assertLog([
-      'Outer: 0',
       'Inner: 0',
-      'Async: 0',
+      'Outer: 0',
+      'Sibling: 0',
       'Inner and outer are consistent',
     ]);
     expect(root).toMatchRenderedOutput(
       <>
-        <span>Outer: 0</span>
         <span>Inner: 0</span>
-        <span>Async: 0</span>
+        <span>Outer: 0</span>
+        <span>Sibling: 0</span>
       </>,
     );
 
     await act(async () => {
       // Update a value both inside and outside the hidden tree. These values
       // must always be consistent.
-      setOuter(1);
-      setInner(1);
-      // In the same render, also hide the offscreen tree.
-      root.render(<App show={false} />);
+      startTransition(() => {
+        setOuter(1);
+        setInner(1);
+        // In the same render, also hide the offscreen tree.
+        root.render(<App show={false} />);
+      });
 
-      await waitForPaint([
+      await waitFor([
         // The outer update will commit, but the inner update is deferred until
         // a later render.
         'Outer: 1',
-
-        // Something suspended. This means we won't commit immediately; there
-        // will be an async gap between render and commit. In this test, we will
-        // use this property to schedule a concurrent update. The fact that
-        // we're using Suspense to schedule a concurrent update is not directly
-        // relevant to the test — we could also use time slicing, but I've
-        // chosen to use Suspense the because implementation details of time
-        // slicing are more volatile.
-        'Suspend! [Async: 1]',
-
-        'Loading...',
       ]);
 
       // Assert that we haven't committed quite yet
       expect(root).toMatchRenderedOutput(
         <>
-          <span>Outer: 0</span>
           <span>Inner: 0</span>
-          <span>Async: 0</span>
+          <span>Outer: 0</span>
+          <span>Sibling: 0</span>
         </>,
       );
 
@@ -520,14 +512,13 @@ describe('ReactOffscreen', () => {
         setInner(2);
       });
 
-      // Commit the previous render.
-      jest.runAllTimers();
+      // Finish rendering and commit the in-progress render.
+      await waitForPaint(['Sibling: 1']);
       expect(root).toMatchRenderedOutput(
         <>
-          <span>Outer: 1</span>
           <span hidden={true}>Inner: 0</span>
-          <span hidden={true}>Async: 0</span>
-          Loading...
+          <span>Outer: 1</span>
+          <span>Sibling: 1</span>
         </>,
       );
 
@@ -536,32 +527,27 @@ describe('ReactOffscreen', () => {
         root.render(<App show={true} />);
       });
       assertLog([
-        'Outer: 1',
-
         // There are two pending updates on Inner, but only the first one
         // is processed, even though they share the same lane. If the second
         // update were erroneously processed, then Inner would be inconsistent
         // with Outer.
         'Inner: 1',
-
-        'Suspend! [Async: 1]',
-        'Loading...',
+        'Outer: 1',
+        'Sibling: 1',
         'Inner and outer are consistent',
       ]);
     });
     assertLog([
-      'Outer: 2',
       'Inner: 2',
-      'Suspend! [Async: 2]',
-      'Loading...',
+      'Outer: 2',
+      'Sibling: 2',
       'Inner and outer are consistent',
     ]);
     expect(root).toMatchRenderedOutput(
       <>
-        <span>Outer: 2</span>
         <span>Inner: 2</span>
-        <span hidden={true}>Async: 0</span>
-        Loading...
+        <span>Outer: 2</span>
+        <span>Sibling: 2</span>
       </>,
     );
   });