Bugfix: Offscreen instance is null during setState

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

@@ -193,8 +193,25 @@ function markUpdateLaneFromFiberToRoot(
     }
 
     if (parent.tag === OffscreenComponent) {
-      const offscreenInstance: OffscreenInstance = parent.stateNode;
-      if (offscreenInstance.isHidden) {
+      // Check if this offscreen boundary is currently hidden.
+      //
+      // The instance may be null if the Offscreen parent was unmounted. Usually
+      // the parent wouldn't be reachable in that case because we disconnect
+      // fibers from the tree when they are deleted. However, there's a weird
+      // edge case where setState is called on a fiber that was interrupted
+      // before it ever mounted. Because it never mounts, it also never gets
+      // deleted. Because it never gets deleted, its return pointer never gets
+      // disconnected. Which means it may be attached to a deleted Offscreen
+      // parent node. (This discovery suggests it may be better for memory usage
+      // if we don't attach the `return` pointer until the commit phase, though
+      // in order to do that we'd need some other way to track the return
+      // pointer during the initial render, like on the stack.)
+      //
+      // This case is always accompanied by a warning, but we still need to
+      // account for it. (There may be other cases that we haven't discovered,
+      // too.)
+      const offscreenInstance: OffscreenInstance | null = parent.stateNode;
+      if (offscreenInstance !== null && offscreenInstance.isHidden) {
         isHidden = true;
       }
     }

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

@@ -7,6 +7,7 @@ let Offscreen;
 let useState;
 let useLayoutEffect;
 let useEffect;
+let startTransition;
 
 describe('ReactOffscreen', () => {
   beforeEach(() => {
@@ -21,6 +22,7 @@ describe('ReactOffscreen', () => {
     useState = React.useState;
     useLayoutEffect = React.useLayoutEffect;
     useEffect = React.useEffect;
+    startTransition = React.startTransition;
   });
 
   function Text(props) {
@@ -591,4 +593,53 @@ describe('ReactOffscreen', () => {
       </>,
     );
   });
+
+  // TODO: Create TestFlag alias for Offscreen
+  // @gate experimental || www
+  it('regression: Offscreen instance is sometimes null during setState', async () => {
+    let setState;
+    function Child() {
+      const [state, _setState] = useState('Initial');
+      setState = _setState;
+      return <Text text={state} />;
+    }
+
+    const root = ReactNoop.createRoot();
+    await act(async () => {
+      root.render(<Offscreen hidden={false} />);
+    });
+    expect(Scheduler).toHaveYielded([]);
+    expect(root).toMatchRenderedOutput(null);
+
+    await act(async () => {
+      // Partially render a component
+      startTransition(() => {
+        root.render(
+          <Offscreen hidden={false}>
+            <Child />
+            <Text text="Sibling" />
+          </Offscreen>,
+        );
+      });
+      expect(Scheduler).toFlushAndYieldThrough(['Initial']);
+
+      // Before it finishes rendering, the whole tree gets deleted
+      ReactNoop.flushSync(() => {
+        root.render(null);
+      });
+
+      // Something attempts to update the never-mounted component. When this
+      // regression test was written, we would walk up the component's return
+      // path and reach an unmounted Offscreen component fiber. Its `stateNode`
+      // would be null because it was nulled out when it was deleted, but there
+      // was no null check before we accessed it. A weird edge case but we must
+      // account for it.
+      expect(() => {
+        setState('Updated');
+      }).toErrorDev(
+        "Can't perform a React state update on a component that hasn't mounted yet",
+      );
+    });
+    expect(root).toMatchRenderedOutput(null);
+  });
 });

scripts/merge-fork/forked-revisions

@@ -1,3 +1,4 @@
+d410f0a1bbb12e972b0e99bb9faea10c7e62894d [FORKED] Bugfix: Offscreen instance is null during setState
 58bb11764bf0bb6db47527a64f693f67cdd3b0bb [FORKED] Check for infinite update loops even if unmounted
 31882b5dd66f34f70d341ea2781cacbe802bf4d5 [FORKED] Bugfix: Revealing a hidden update
 17691acc071d56261d43c3cf183f287d983baa9b [FORKED] Don't update childLanes until after current render