[DevTools] Stop mounting empty roots (facebook#34467)

Author: eps1lon

packages/react-devtools-shared/src/__tests__/store-test.js

@@ -2489,7 +2489,7 @@ describe('Store', () => {
       withErrorsOrWarningsIgnored(['test-only:'], async () => {
         await act(() => render(<React.Fragment />));
       });
-      expect(store).toMatchInlineSnapshot(`[root]`);
+      expect(store).toMatchInlineSnapshot(``);
       expect(store.componentWithErrorCount).toBe(0);
       expect(store.componentWithWarningCount).toBe(0);
     });
@@ -3083,6 +3083,9 @@ describe('Store', () => {
 
   it('should handle an empty root', async () => {
     await actAsync(() => render(null));
+    expect(store).toMatchInlineSnapshot(``);
+
+    await actAsync(() => render(<span />));
     expect(store).toMatchInlineSnapshot(`[root]`);
   });
 });

packages/react-devtools-shared/src/backend/fiber/renderer.js

@@ -5322,12 +5322,12 @@ export function attach(
     root: FiberRoot,
     priorityLevel: void | number,
   ) {
-    const current = root.current;
+    const nextFiber = root.current;
 
     let prevFiber: null | Fiber = null;
     let rootInstance = rootToFiberInstanceMap.get(root);
     if (!rootInstance) {
-      rootInstance = createFiberInstance(current);
+      rootInstance = createFiberInstance(nextFiber);
       rootToFiberInstanceMap.set(root, rootInstance);
       idToDevToolsInstanceMap.set(rootInstance.id, rootInstance);
     } else {
@@ -5366,30 +5366,25 @@ export function attach(
       };
     }
 
-    if (prevFiber !== null) {
-      // TODO: relying on this seems a bit fishy.
-      const wasMounted =
-        prevFiber.memoizedState != null &&
-        prevFiber.memoizedState.element != null;
-      const isMounted =
-        current.memoizedState != null && current.memoizedState.element != null;
-      if (!wasMounted && isMounted) {
-        // Mount a new root.
-        setRootPseudoKey(currentRoot.id, current);
-        mountFiberRecursively(current, false);
-      } else if (wasMounted && isMounted) {
-        // Update an existing root.
-        updateFiberRecursively(rootInstance, current, prevFiber, false);
-      } else if (wasMounted && !isMounted) {
-        // Unmount an existing root.
-        unmountInstanceRecursively(rootInstance);
-        removeRootPseudoKey(currentRoot.id);
-        rootToFiberInstanceMap.delete(root);
-      }
-    } else {
+    const nextIsMounted = nextFiber.child !== null;
+    const prevWasMounted = prevFiber !== null && prevFiber.child !== null;
+    if (!prevWasMounted && nextIsMounted) {
       // Mount a new root.
-      setRootPseudoKey(currentRoot.id, current);
-      mountFiberRecursively(current, false);
+      setRootPseudoKey(currentRoot.id, nextFiber);
+      mountFiberRecursively(nextFiber, false);
+    } else if (prevWasMounted && nextIsMounted) {
+      if (prevFiber === null) {
+        throw new Error(
+          'Expected a previous Fiber when updating an existing root.',
+        );
+      }
+      // Update an existing root.
+      updateFiberRecursively(rootInstance, nextFiber, prevFiber, false);
+    } else if (prevWasMounted && !nextIsMounted) {
+      // Unmount an existing root.
+      unmountInstanceRecursively(rootInstance);
+      removeRootPseudoKey(currentRoot.id);
+      rootToFiberInstanceMap.delete(root);
     }
 
     if (isProfiling && isProfilingSupported) {