Added test repro for FB error

Author: Brian Vaughn

packages/react-dom/src/__tests__/ReactDOMFiber-test.js

@@ -1280,4 +1280,40 @@ describe('ReactDOMFiber', () => {
     );
     expect(didCallOnChange).toBe(true);
   });
+
+  it('unmounted legacy roots should never clear newer root content from a container', () => {
+    const ref = React.createRef();
+
+    function OldApp() {
+      const hideOnFocus = () => {
+        // This app unmounts itself inside of a focus event.
+        ReactDOM.unmountComponentAtNode(container);
+      };
+
+      return (
+        <button onFocus={hideOnFocus} ref={ref}>
+          old
+        </button>
+      );
+    }
+
+    function NewApp() {
+      return (
+        <button tabIndex={0} ref={ref}>
+          new
+        </button>
+      );
+    }
+
+    ReactDOM.render(<OldApp />, container);
+    ref.current.focus();
+
+    ReactDOM.render(<NewApp />, container);
+
+    // Calling focus again will flush previously scheduled discerete work for the old root-
+    // but this should not clear out the newly mounted app.
+    ref.current.focus();
+
+    expect(container.textContent).toBe('new');
+  });
 });