[Fizz] If we haven't painted yet, wait to reveal everything until next paint (#34230)

Before the first rAF, we don't know if there has been other paints
before this and if so when. (We could get from performance observer.) We
can assume that it's not earlier than 0 so we used delay up until the
throttle time starting from zero but if the first paint is about to
happen that can be very soon after.

Instead, this reveals it during the next paint which should let us be
able to get into the first paint. If we can trust `rel="expect"` to have
done its thing we should schedule our raf before first paint but ofc
browsers can cheat and paint earlier if they want to.

If we're wrong, this is at least more batched than doing it
synchronously. However it will mean that things might get more flashy
than it should be if it would've been throttled. An alternative would be
to always throttle first reveal.

Author: sebmarkbage

fixtures/ssr/src/components/LargeContent.js

@@ -1,8 +1,4 @@
-import React, {
-  Fragment,
-  Suspense,
-  unstable_SuspenseList as SuspenseList,
-} from 'react';
+import React, {Suspense, unstable_SuspenseList as SuspenseList} from 'react';
 
 export default function LargeContent() {
   return (

packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js

@@ -420,21 +420,24 @@ export function completeBoundary(suspenseBoundaryID, contentID) {
   if (window['$RB'].length === 2) {
     // This is the first time we've pushed to the batch. We need to schedule a callback
     // to flush the batch. This is delayed by the throttle heuristic.
-    const globalMostRecentFallbackTime =
-      typeof window['$RT'] !== 'number' ? 0 : window['$RT'];
-    const currentTime = performance.now();
-    const msUntilTimeout =
-      // If the throttle would make us miss the target metric, then shorten the throttle.
-      // performance.now()'s zero value is assumed to be the start time of the metric.
-      currentTime < TARGET_VANITY_METRIC &&
-      currentTime > TARGET_VANITY_METRIC - FALLBACK_THROTTLE_MS
-        ? TARGET_VANITY_METRIC - currentTime
-        : // Otherwise it's throttled starting from last commit time.
-          globalMostRecentFallbackTime + FALLBACK_THROTTLE_MS - currentTime;
-    // We always schedule the flush in a timer even if it's very low or negative to allow
-    // for multiple completeBoundary calls that are already queued to have a chance to
-    // make the batch.
-    setTimeout(window['$RV'].bind(null, window['$RB']), msUntilTimeout);
+    if (typeof window['$RT'] !== 'number') {
+      // If we haven't had our rAF callback yet, schedule everything for the first paint.
+      requestAnimationFrame(window['$RV'].bind(null, window['$RB']));
+    } else {
+      const currentTime = performance.now();
+      const msUntilTimeout =
+        // If the throttle would make us miss the target metric, then shorten the throttle.
+        // performance.now()'s zero value is assumed to be the start time of the metric.
+        currentTime < TARGET_VANITY_METRIC &&
+        currentTime > TARGET_VANITY_METRIC - FALLBACK_THROTTLE_MS
+          ? TARGET_VANITY_METRIC - currentTime
+          : // Otherwise it's throttled starting from last commit time.
+            window['$RT'] + FALLBACK_THROTTLE_MS - currentTime;
+      // We always schedule the flush in a timer even if it's very low or negative to allow
+      // for multiple completeBoundary calls that are already queued to have a chance to
+      // make the batch.
+      setTimeout(window['$RV'].bind(null, window['$RB']), msUntilTimeout);
+    }
   }
 }
 

packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js

@@ -40,6 +40,7 @@ describe('ReactDOMFizzStaticBrowser', () => {
 
     // We need the mocked version of setTimeout inside the document.
     window.setTimeout = setTimeout;
+    window.requestAnimationFrame = setTimeout;
 
     patchMessageChannel();
     serverAct = require('internal-test-utils').serverAct;