Flush legacy passive effects at beginning of event

Fixes test added in previous commit.

Author: acdlite

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -1082,6 +1082,15 @@ export function flushSyncWithoutWarningIfAlreadyRendering<A, R>(
   fn: A => R,
   a: A,
 ): R {
+  // In legacy mode, we flush pending passive effects at the beginning of the
+  // next event, not at the end of the previous one.
+  if (
+    rootWithPendingPassiveEffects !== null &&
+    rootWithPendingPassiveEffects.tag === LegacyRoot
+  ) {
+    flushPassiveEffects();
+  }
+
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
 

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -1082,6 +1082,15 @@ export function flushSyncWithoutWarningIfAlreadyRendering<A, R>(
   fn: A => R,
   a: A,
 ): R {
+  // In legacy mode, we flush pending passive effects at the beginning of the
+  // next event, not at the end of the previous one.
+  if (
+    rootWithPendingPassiveEffects !== null &&
+    rootWithPendingPassiveEffects.tag === LegacyRoot
+  ) {
+    flushPassiveEffects();
+  }
+
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
 

packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js

@@ -1795,10 +1795,11 @@ describe('ReactHooksWithNoopRenderer', () => {
           return <Text text={'Count: ' + count} />;
         }
         await act(async () => {
-          ReactNoop.renderLegacySyncRoot(<Counter count={0} />);
+          ReactNoop.flushSync(() => {
+            ReactNoop.renderLegacySyncRoot(<Counter count={0} />);
+          });
 
           // Even in legacy mode, effects are deferred until after paint
-          ReactNoop.flushSync();
           expect(Scheduler).toHaveYielded(['Count: (empty)']);
           expect(ReactNoop.getChildren()).toEqual([span('Count: (empty)')]);
         });