From fd23be0e2b9ea6d4d569a8e236fb05ac8cd2aec0 Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Mon, 12 May 2025 22:30:52 -0400
Subject: [PATCH] Always flush Default priority in the microtask if a
 Transition was scheduled too

---
 packages/react-reconciler/src/ReactFiberRootScheduler.js  | 8 ++++++++
 .../src/__tests__/ReactDefaultTransitionIndicator-test.js | 5 +----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/packages/react-reconciler/src/ReactFiberRootScheduler.js b/packages/react-reconciler/src/ReactFiberRootScheduler.js
index 7daed077eee..0bcf1d580c6 100644
--- a/packages/react-reconciler/src/ReactFiberRootScheduler.js
+++ b/packages/react-reconciler/src/ReactFiberRootScheduler.js
@@ -26,6 +26,7 @@ import {
   NoLane,
   NoLanes,
   SyncLane,
+  DefaultLane,
   getHighestPriorityLane,
   getNextLanes,
   includesSyncLane,
@@ -261,6 +262,13 @@ function processRootScheduleInMicrotask() {
       // render it synchronously anyway. We do this during a popstate event to
       // preserve the scroll position of the previous page.
       syncTransitionLanes = currentEventTransitionLane;
+    } else if (enableDefaultTransitionIndicator) {
+      // If we have a Transition scheduled by this event it might be paired
+      // with Default lane scheduled loading indicators. To unbatch it from
+      // other events later on, flush it early to determine whether it
+      // rendered an indicator. This ensures that setState in default priority
+      // event doesn't trigger onDefaultTransitionIndicator.
+      syncTransitionLanes = DefaultLane;
     }
   }
 
diff --git a/packages/react-reconciler/src/__tests__/ReactDefaultTransitionIndicator-test.js b/packages/react-reconciler/src/__tests__/ReactDefaultTransitionIndicator-test.js
index bc5c3ec6695..71848056034 100644
--- a/packages/react-reconciler/src/__tests__/ReactDefaultTransitionIndicator-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactDefaultTransitionIndicator-test.js
@@ -109,10 +109,7 @@ describe('ReactDefaultTransitionIndicator', () => {
     expect(root).toMatchRenderedOutput(<div>Hi</div>);
 
     await act(() => {
-      // TODO: This should not require a discrete update ideally but work for default too.
-      ReactNoop.discreteUpdates(() => {
-        update('Loading...');
-      });
+      update('Loading...');
       React.startTransition(() => {
         update('');
         root.render(<App>{promiseB}</App>);