Fix unwinding context during selective hydration

Author: tyao1

packages/react-dom/src/__tests__/ReactDOMServerSelectiveHydration-test.internal.js

@@ -1826,7 +1826,7 @@ describe('ReactDOMServerSelectiveHydration', () => {
       TODO_scheduleIdleDOMSchedulerTask(() => {
         root.render(<App a="AA" />);
       });
-      expect(Scheduler).toFlushAndYieldThrough(['App', 'AA', 'DefaultContext']);
+      expect(Scheduler).toFlushAndYieldThrough(['App', 'A']);
 
       dispatchClickEvent(spanA);
       expect(Scheduler).toHaveYielded(['A']);

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -2245,8 +2245,9 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
             // Selective hydration. An update flowed into a dehydrated tree.
             // Interrupt the current render so the work loop can switch to the
             // hydration lane.
-            workInProgress = null;
             workInProgressRootExitStatus = RootDidNotComplete;
+            // `workInProgress` is not set to `null` to allow unwinding in
+            // `prepareFreshStack`.
             break outer;
           }
           default: {