Prevent stale legacy root from clearing a container (DRAFT) (#18792)

* Don't clear a container because of a stale legacy root

* Added test repro for FB error

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

@@ -1280,4 +1280,36 @@ 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 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');
+  });
 });

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

@@ -687,8 +687,21 @@ function completeWork(
           // It's also safe to do for updates too, because current.child would only be null
           // if the previous render was null (so the the container would already be empty).
           //
-          // The additional root.hydrate check is required for hydration in legacy mode with no fallback.
-          workInProgress.effectTag |= Snapshot;
+          // The additional root.hydrate check above is required for hydration in legacy mode with no fallback.
+          //
+          // The root container check below also avoids a potential legacy mode problem
+          // where unmounting from a container then rendering into it again
+          // can sometimes cause the container to be cleared after the new render.
+          const containerInfo = fiberRoot.containerInfo;
+          const legacyRootContainer =
+            containerInfo == null ? null : containerInfo._reactRootContainer;
+          if (
+            legacyRootContainer == null ||
+            legacyRootContainer._internalRoot == null ||
+            legacyRootContainer._internalRoot === fiberRoot
+          ) {
+            workInProgress.effectTag |= Snapshot;
+          }
         }
       }
       updateHostContainer(workInProgress);

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

@@ -685,8 +685,21 @@ function completeWork(
           // It's also safe to do for updates too, because current.child would only be null
           // if the previous render was null (so the the container would already be empty).
           //
-          // The additional root.hydrate check is required for hydration in legacy mode with no fallback.
-          workInProgress.effectTag |= Snapshot;
+          // The additional root.hydrate check above is required for hydration in legacy mode with no fallback.
+          //
+          // The root container check below also avoids a potential legacy mode problem
+          // where unmounting from a container then rendering into it again
+          // can sometimes cause the container to be cleared after the new render.
+          const containerInfo = fiberRoot.containerInfo;
+          const legacyRootContainer =
+            containerInfo == null ? null : containerInfo._reactRootContainer;
+          if (
+            legacyRootContainer == null ||
+            legacyRootContainer._internalRoot == null ||
+            legacyRootContainer._internalRoot === fiberRoot
+          ) {
+            workInProgress.effectTag |= Snapshot;
+          }
         }
       }
       updateHostContainer(workInProgress);