[DevTools] Handle fallback unmount in Suspense update path (#34199)

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

@@ -2696,4 +2696,83 @@ describe('Store', () => {
               <ClientComponent key="D">
       `);
   });
+
+  // @reactVersion >= 18.0
+  it('can reconcile Suspense in fallback positions', async () => {
+    let resolveFallback;
+    const fallbackPromise = new Promise(resolve => {
+      resolveFallback = resolve;
+    });
+    let resolveContent;
+    const contentPromise = new Promise(resolve => {
+      resolveContent = resolve;
+    });
+
+    function Component({children, promise}) {
+      if (promise) {
+        React.use(promise);
+      }
+      return <div>{children}</div>;
+    }
+
+    await actAsync(() =>
+      render(
+        <React.Suspense
+          name="content"
+          fallback={
+            <React.Suspense
+              name="fallback"
+              fallback={
+                <Component key="fallback-fallback">
+                  Loading fallback...
+                </Component>
+              }>
+              <Component key="fallback-content" promise={fallbackPromise}>
+                Loading...
+              </Component>
+            </React.Suspense>
+          }>
+          <Component key="content" promise={contentPromise}>
+            done
+          </Component>
+        </React.Suspense>,
+      ),
+    );
+
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+        ▾ <Suspense name="content">
+          ▾ <Suspense name="fallback">
+              <Component key="fallback-fallback">
+      [shell]
+        <Suspense name="content" rects={null}>
+        <Suspense name="fallback" rects={null}>
+    `);
+
+    await actAsync(() => {
+      resolveFallback();
+    });
+
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+        ▾ <Suspense name="content">
+          ▾ <Suspense name="fallback">
+              <Component key="fallback-content">
+      [shell]
+        <Suspense name="content" rects={null}>
+        <Suspense name="fallback" rects={[{x:1,y:2,width:10,height:1}]}>
+    `);
+
+    await actAsync(() => {
+      resolveContent();
+    });
+
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+        ▾ <Suspense name="content">
+            <Component key="content">
+      [shell]
+        <Suspense name="content" rects={[{x:1,y:2,width:4,height:1}]}>
+    `);
+  });
 });

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

@@ -4736,26 +4736,30 @@ export function attach(
         );
 
         shouldMeasureSuspenseNode = false;
-        if (nextFallbackFiber !== null) {
+        if (prevFallbackFiber !== null || nextFallbackFiber !== null) {
           const fallbackStashedSuspenseParent = reconcilingParentSuspenseNode;
           const fallbackStashedSuspensePrevious =
             previouslyReconciledSiblingSuspenseNode;
           const fallbackStashedSuspenseRemaining =
             remainingReconcilingChildrenSuspenseNodes;
           // Next, we'll pop back out of the SuspenseNode that we added above and now we'll
-          // reconcile the fallback, reconciling anything by inserting into the parent SuspenseNode.
+          // reconcile the fallback, reconciling anything in the context of the parent SuspenseNode.
           // Since the fallback conceptually blocks the parent.
           reconcilingParentSuspenseNode = stashedSuspenseParent;
           previouslyReconciledSiblingSuspenseNode = stashedSuspensePrevious;
           remainingReconcilingChildrenSuspenseNodes = stashedSuspenseRemaining;
           try {
-            updateFlags |= updateVirtualChildrenRecursively(
-              nextFallbackFiber,
-              null,
-              prevFallbackFiber,
-              traceNearestHostComponentUpdate,
-              0,
-            );
+            if (nextFallbackFiber === null) {
+              unmountRemainingChildren();
+            } else {
+              updateFlags |= updateVirtualChildrenRecursively(
+                nextFallbackFiber,
+                null,
+                prevFallbackFiber,
+                traceNearestHostComponentUpdate,
+                0,
+              );
+            }
           } finally {
             reconcilingParentSuspenseNode = fallbackStashedSuspenseParent;
             previouslyReconciledSiblingSuspenseNode =
@@ -4763,7 +4767,8 @@ export function attach(
             remainingReconcilingChildrenSuspenseNodes =
               fallbackStashedSuspenseRemaining;
           }
-        } else if (nextFiber.memoizedState === null) {
+        }
+        if (nextFiber.memoizedState === null) {
           // Measure this Suspense node in case it changed. We don't update the rect while
           // we're inside a disconnected subtree nor if we are the Suspense boundary that
           // is suspended. This lets us keep the rectangle of the displayed content while