Regression test: Interleaved update race condition

Demonstrates the bug reported in facebook#24350.

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

@@ -140,4 +140,52 @@ describe('ReactInterleavedUpdates', () => {
     expect(Scheduler).toHaveYielded([2, 2, 2]);
     expect(root).toMatchRenderedOutput('222');
   });
+
+  test('regression for #24350: does not add to main update queue until interleaved update queue has been cleared', async () => {
+    let setStep;
+    function App() {
+      const [step, _setState] = useState(0);
+      setStep = _setState;
+      return (
+        <>
+          <Text text={'A' + step} />
+          <Text text={'B' + step} />
+          <Text text={'C' + step} />
+        </>
+      );
+    }
+
+    const root = ReactNoop.createRoot();
+    await act(async () => {
+      root.render(<App />);
+    });
+    expect(Scheduler).toHaveYielded(['A0', 'B0', 'C0']);
+    expect(root).toMatchRenderedOutput('A0B0C0');
+
+    await act(async () => {
+      // Start the render phase.
+      startTransition(() => {
+        setStep(1);
+      });
+      expect(Scheduler).toFlushAndYieldThrough(['A1', 'B1']);
+
+      // Schedule an interleaved update. This gets placed on a special queue.
+      startTransition(() => {
+        setStep(2);
+      });
+
+      // Finish rendering the first update.
+      expect(Scheduler).toFlushUntilNextPaint(['C1']);
+
+      // Schedule another update. (In the regression case, this was treated
+      // as a normal, non-interleaved update and it was inserted into the queue
+      // before the interleaved one was processed.)
+      startTransition(() => {
+        setStep(3);
+      });
+    });
+    // The last update should win.
+    expect(Scheduler).toHaveYielded(['A3', 'B3', 'C3']);
+    expect(root).toMatchRenderedOutput('A3B3C3');
+  });
 });