Revert "[Re-land] Make prerendering always non-blocking (facebook#31268)" (facebook#31355)

This reverts commit 6c4bbc7.

It looked like the bug we found on the original land was related to
broken product code. But through landing facebook#31268 we found additional bugs
internally. Since disabling the feature flag does not fix the bugs, we
have to revert again to unblock the sync. We can continue to debug with
our internal build.

Author: jackpope

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(gate('enableSiblingPrerendering') ? ['Suspend! [/path/b]'] : []);
+    assertLog([]);
 
     await act(async () => {
       resolvePromise();

packages/react-reconciler/src/ReactFiberLane.js

@@ -765,14 +765,12 @@ export function markRootSuspended(
   root: FiberRoot,
   suspendedLanes: Lanes,
   spawnedLane: Lane,
-  didAttemptEntireTree: boolean,
+  didSkipSuspendedSiblings: 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 && didAttemptEntireTree) {
+  if (enableSiblingPrerendering && !didSkipSuspendedSiblings) {
     // 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

@@ -18,7 +18,6 @@ import {
   disableSchedulerTimeoutInWorkLoop,
   enableProfilerTimer,
   enableProfilerNestedUpdatePhase,
-  enableSiblingPrerendering,
 } from 'shared/ReactFeatureFlags';
 import {
   NoLane,
@@ -30,7 +29,6 @@ import {
   markStarvedLanesAsExpired,
   claimNextTransitionLane,
   getNextLanesToFlushSync,
-  checkIfRootIsPrerendering,
 } from './ReactFiberLane';
 import {
   CommitContext,
@@ -208,10 +206,7 @@ function flushSyncWorkAcrossRoots_impl(
               ? workInProgressRootRenderLanes
               : NoLanes,
           );
-          if (
-            includesSyncLane(nextLanes) &&
-            !checkIfRootIsPrerendering(root, nextLanes)
-          ) {
+          if (includesSyncLane(nextLanes)) {
             // This root has pending sync work. Flush it now.
             didPerformSomeWork = true;
             performSyncWorkOnRoot(root, nextLanes);
@@ -346,13 +341,7 @@ function scheduleTaskForRootDuringMicrotask(
   }
 
   // Schedule a new callback in the host environment.
-  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))
-  ) {
+  if (includesSyncLane(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) {
@@ -386,10 +375,9 @@ 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;