[Re-land] Make prerendering always non-blocking (#31268)

Follows #31238

___

This is a partial re-land of
#31056. We saw breakages surface
after the original land and had to revert. Now that they've been fixed,
let's try this again. This time we'll split up the commits to give us
more control of testing and rollout internally.

Original PR: #31056
Original Commit:
4c71025
Revert PR: #31080

Commit description:

> When a synchronous update suspends, and we prerender the siblings, the
prerendering should be non-blocking so that we can immediately restart
once the data arrives.
>
> This happens automatically when there's a Suspense boundary, because
we immediately commit the boundary and then proceed to a Retry render,
which are always concurrent. When there's not a Suspense boundary, there
is no Retry, so we need to take care to switch from the synchronous work
loop to the concurrent one, to enable time slicing.

Co-authored-by: Andrew Clark <git@andrewclark.io>

packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.js

@@ -744,7 +744,7 @@ describe('ReactDOMFiberAsync', () => {
       // Because it suspended, it remains on the current path
       expect(div.textContent).toBe('/path/a');
     });
-    assertLog([]);
+    assertLog(gate('enableSiblingPrerendering') ? ['Suspend! [/path/b]'] : []);
 
     await act(async () => {
       resolvePromise();

packages/react-reconciler/src/ReactFiberLane.js

@@ -765,12 +765,14 @@ export function markRootSuspended(
   root: FiberRoot,
   suspendedLanes: Lanes,
   spawnedLane: Lane,
-  didSkipSuspendedSiblings: boolean,
+  didAttemptEntireTree: boolean,
 ) {
+  // TODO: Split this into separate functions for marking the root at the end of
+  // a render attempt versus suspending while the root is still in progress.
   root.suspendedLanes |= suspendedLanes;
   root.pingedLanes &= ~suspendedLanes;
 
-  if (enableSiblingPrerendering && !didSkipSuspendedSiblings) {
+  if (enableSiblingPrerendering && didAttemptEntireTree) {
     // Mark these lanes as warm so we know there's nothing else to work on.
     root.warmLanes |= suspendedLanes;
   } else {

packages/react-reconciler/src/ReactFiberRootScheduler.js

@@ -18,6 +18,7 @@ import {
   disableSchedulerTimeoutInWorkLoop,
   enableProfilerTimer,
   enableProfilerNestedUpdatePhase,
+  enableSiblingPrerendering,
 } from 'shared/ReactFeatureFlags';
 import {
   NoLane,
@@ -29,6 +30,7 @@ import {
   markStarvedLanesAsExpired,
   claimNextTransitionLane,
   getNextLanesToFlushSync,
+  checkIfRootIsPrerendering,
 } from './ReactFiberLane';
 import {
   CommitContext,
@@ -206,7 +208,10 @@ function flushSyncWorkAcrossRoots_impl(
               ? workInProgressRootRenderLanes
               : NoLanes,
           );
-          if (includesSyncLane(nextLanes)) {
+          if (
+            includesSyncLane(nextLanes) &&
+            !checkIfRootIsPrerendering(root, nextLanes)
+          ) {
             // This root has pending sync work. Flush it now.
             didPerformSomeWork = true;
             performSyncWorkOnRoot(root, nextLanes);
@@ -341,7 +346,13 @@ function scheduleTaskForRootDuringMicrotask(
   }
 
   // Schedule a new callback in the host environment.
-  if (includesSyncLane(nextLanes)) {
+  if (
+    includesSyncLane(nextLanes) &&
+    // If we're prerendering, then we should use the concurrent work loop
+    // even if the lanes are synchronous, so that prerendering never blocks
+    // the main thread.
+    !(enableSiblingPrerendering && checkIfRootIsPrerendering(root, nextLanes))
+  ) {
     // Synchronous work is always flushed at the end of the microtask, so we
     // don't need to schedule an additional task.
     if (existingCallbackNode !== null) {
@@ -375,9 +386,10 @@ function scheduleTaskForRootDuringMicrotask(
 
     let schedulerPriorityLevel;
     switch (lanesToEventPriority(nextLanes)) {
+      // Scheduler does have an "ImmediatePriority", but now that we use
+      // microtasks for sync work we no longer use that. Any sync work that
+      // reaches this path is meant to be time sliced.
       case DiscreteEventPriority:
-        schedulerPriorityLevel = ImmediateSchedulerPriority;
-        break;
       case ContinuousEventPriority:
         schedulerPriorityLevel = UserBlockingSchedulerPriority;
         break;