Exclude hydration from nested update check

If a commit was the result of selective hydration, we should not
increment the nested update count, because those renders conceptually
are not updates.

Ideally, they wouldn't even be in a separate commit — we should be able
to hydrate a tree and apply an update on top of it within the same
render phase. We could do this once we implement resumable
context stacks.

packages/react-reconciler/src/ReactFiberLane.js

@@ -46,7 +46,9 @@ export const InputContinuousLane: Lane = /*             */ 0b0000000000000000000
 export const DefaultHydrationLane: Lane = /*            */ 0b0000000000000000000000000010000;
 export const DefaultLane: Lane = /*                     */ 0b0000000000000000000000000100000;
 
-export const SyncUpdateLanes: Lane = /*                */ 0b0000000000000000000000000101010;
+export const SyncUpdateLanes: Lane = enableUnifiedSyncLane
+  ? SyncLane | InputContinuousLane | DefaultLane
+  : SyncLane;
 
 const TransitionHydrationLane: Lane = /*                */ 0b0000000000000000000000001000000;
 const TransitionLanes: Lanes = /*                       */ 0b0000000011111111111111110000000;

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -154,6 +154,7 @@ import {
   includesOnlyNonUrgentLanes,
   includesSomeLane,
   OffscreenLane,
+  SyncUpdateLanes,
 } from './ReactFiberLane';
 import {
   DiscreteEventPriority,
@@ -2932,7 +2933,17 @@ function commitRootImpl(
 
   // Read this again, since a passive effect might have updated it
   remainingLanes = root.pendingLanes;
-  if (includesSyncLane(remainingLanes)) {
+
+  // Check if this render scheduled a cascading synchronous update. This is a
+  // heurstic to detect infinite update loops. We are intentionally excluding
+  // hydration lanes in this check, because render triggered by selective
+  // hydration is conceptually not an update.
+  if (
+    // Was the finished render the result of a sync update?
+    includesSomeLane(lanes, SyncUpdateLanes) &&
+    // Did is schedule another sync update?
+    includesSomeLane(remainingLanes, SyncUpdateLanes)
+  ) {
     if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
       markNestedUpdateScheduled();
     }