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;
@@ -84,6 +86,11 @@ export const IdleLane: Lane = /*                        */ 0b0100000000000000000
 
 export const OffscreenLane: Lane = /*                   */ 0b1000000000000000000000000000000;
 
+// Any lane that might schedule an update. This is used to detect infinite
+// update loops, so it doesn't include hydration lanes or retries.
+export const UpdateLanes =
+  SyncLane | InputContinuousLane | DefaultLane | TransitionLanes;
+
 // This function is used for the experimental timeline (react-devtools-timeline)
 // It should be kept in sync with the Lanes values above.
 export function getLabelForLane(lane: Lane): string | void {

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -154,6 +154,8 @@ import {
   includesOnlyNonUrgentLanes,
   includesSomeLane,
   OffscreenLane,
+  SyncUpdateLanes,
+  UpdateLanes,
 } from './ReactFiberLane';
 import {
   DiscreteEventPriority,
@@ -2932,7 +2934,17 @@ 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 an update (not hydration)?
+    includesSomeLane(lanes, UpdateLanes) &&
+    // Did it schedule a sync update?
+    includesSomeLane(remainingLanes, SyncUpdateLanes)
+  ) {
     if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
       markNestedUpdateScheduled();
     }