fix(web): opening a deleted thread from the timeline stays on the current thread

apps/web/src/components/ChatView.tsx

@@ -379,6 +379,7 @@ import {
   useThreadRefs,
   useThreadVisibleTurnItems,
   useThreadCheckpointSummaries,
+  readThreadShell,
   waitForThreadShell,
 } from "../state/entities";
 import { environmentShell } from "../state/shell";
@@ -563,6 +564,9 @@ const EMPTY_FEEDBACK_SUBMISSIONS: ReadonlyArray<CodexFeedbackSubmission> = [];
 // second, and every server-side visit is a full command dispatch plus a
 // broadcast to all shell subscribers.
 const VISIT_DISPATCH_THROTTLE_MS = 10_000;
+// Long enough for a just-launched child's shell to sync. A thread with no shell waits
+// this long before the deleted toast.
+const RELATED_THREAD_SHELL_WAIT_MS = 1_000;
 const EMPTY_PROVIDER_SKILLS: ServerProvider["skills"] = [];
 function useDraftHeroLayoutTransition(
   isDraftHeroState: boolean,
@@ -5104,10 +5108,23 @@ export default function ChatView(props: ChatViewProps) {
     }
   };
   const handleStopBackgroundWorkStable = useStableCallback(handleStopBackgroundWork);
-  const onOpenRelatedThread = (threadId: ThreadId) => {
+  const onOpenRelatedThread = async (threadId: ThreadId) => {
+    const threadRef = scopeThreadRef(environmentId, threadId);
+    const environmentLive =
+      appAtomRegistry.get(environmentShell.stateValueAtom(environmentId)).status === "live";
+    // A just-launched child can reach this timeline before its shell syncs.
+    if (environmentLive && readThreadShell(threadRef) === null) {
+      await waitForThreadShell(threadRef, RELATED_THREAD_SHELL_WAIT_MS);
+    }
+    const shell = readThreadShell(threadRef);
+    // The thread route sends a missing thread to the start page, so stay here instead.
+    if (shell === null ? environmentLive : shell.deletedAt != null) {
+      toastManager.add({ type: "info", title: "This thread was deleted" });
+      return;
+    }
     void navigate({
       to: "/$environmentId/$threadId",
-      params: buildThreadRouteParams(scopeThreadRef(environmentId, threadId)),
+      params: buildThreadRouteParams(threadRef),
     });
   };
   const onOpenRelatedThreadStable = useStableCallback(onOpenRelatedThread);