[DevTools] Fix crash when revealing stable, filtered <Activity> children (#35734)

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

@@ -3572,4 +3572,48 @@ describe('Store', () => {
           <Suspense name="inner" rects={[{x:1,y:2,width:14,height:1}]}>
     `);
   });
+
+  // @reactVersion >= 19
+  it('can reconcile newly visible Activity with filtered, stable children', async () => {
+    const Activity = React.Activity || React.unstable_Activity;
+
+    function IgnoreMe({children}) {
+      return children;
+    }
+
+    function Component({children}) {
+      return <div>{children}</div>;
+    }
+
+    await actAsync(
+      async () =>
+        (store.componentFilters = [createDisplayNameFilter('^IgnoreMe', true)]),
+    );
+
+    const children = (
+      <IgnoreMe>
+        <Component key="left">Left</Component>
+      </IgnoreMe>
+    );
+
+    await actAsync(() => {
+      render(<Activity mode="hidden">{children}</Activity>);
+    });
+
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+          <Activity>
+    `);
+
+    await actAsync(() => {
+      render(<Activity mode="visible">{children}</Activity>);
+    });
+
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+        ▾ <Activity>
+          ▾ <Component key="left">
+              <div>
+    `);
+  });
 });

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

@@ -5416,6 +5416,17 @@ export function attach(
           // We're hiding the children. Remove them from the Frontend
           unmountRemainingChildren();
         }
+      } else if (prevWasHidden && !nextIsHidden) {
+        // Since we don't mount hidden children and unmount children when hiding,
+        // we need to enter the mount path when revealing.
+        const nextChildSet = nextFiber.child;
+        if (nextChildSet !== null) {
+          mountChildrenRecursively(
+            nextChildSet,
+            traceNearestHostComponentUpdate,
+          );
+          updateFlags |= ShouldResetChildren | ShouldResetSuspenseChildren;
+        }
       } else if (
         nextFiber.tag === SuspenseComponent &&
         OffscreenComponent !== -1 &&