fix(seer-explorer): Simplify optimistic UI clearing logic

Remove baselineSignature comparison and timeout in favor of directly
checking whether the server has the expected user block at the insert
index with matching content, then whether an assistant block follows.

Author: Mihir-Mavalankar

static/app/views/seerExplorer/hooks/useSeerExplorer.tsx

@@ -148,8 +148,6 @@ export const useSeerExplorer = () => {
   const [optimistic, setOptimistic] = useState<{
     assistantBlockId: string;
     assistantContent: string;
-    baselineSignature: string;
-    createdAt: number;
     insertIndex: number;
     userBlockId: string;
     userQuery: string;
@@ -210,16 +208,6 @@ export const useSeerExplorer = () => {
         deletedFromIndex ?? (apiData?.session?.blocks.length || 0);
       const calculatedInsertIndex = insertIndex ?? effectiveMessageLength;
 
-      // Record current real blocks signature to know when to clear optimistic UI
-      const baselineSignature = JSON.stringify(
-        (apiData?.session?.blocks || []).map(b => [
-          b.id,
-          b.message.role,
-          b.message.content,
-          !!b.loading,
-        ])
-      );
-
       // Generate deterministic block IDs matching backend logic
       // Backend generates: `{prefix}-{index}-{content[:16].replace(' ', '-')}`
       const generateBlockId = (prefix: string, content: string, index: number) => {
@@ -236,8 +224,6 @@ export const useSeerExplorer = () => {
       setOptimistic({
         insertIndex: calculatedInsertIndex,
         userQuery: query,
-        baselineSignature,
-        createdAt: Date.now(),
         userBlockId: generateBlockId('user', query, calculatedInsertIndex),
         assistantBlockId: generateBlockId(
           'loading',
@@ -457,53 +443,34 @@ export const useSeerExplorer = () => {
     return sessionData;
   }, [sessionData, deletedFromIndex, optimistic, runId]);
 
-  // Clear optimistic blocks once the server has a real assistant response.
-  // The server acknowledges the user message before the Celery worker produces
-  // an assistant block, so we keep the optimistic "Thinking" block visible
-  // until a real assistant block appears after the insert point (or a 30s timeout).
+  // Clear optimistic blocks once the server has persisted the user message
+  // and produced a real assistant response after the insert point.
   useEffect(() => {
     if (!optimistic) {
       return undefined;
     }
 
-    const isTimedOut = Date.now() - optimistic.createdAt > 30_000;
+    const serverBlocks = apiData?.session?.blocks || [];
+    const blockAtInsert = serverBlocks[optimistic.insertIndex];
 
-    if (isTimedOut) {
-      setOptimistic(null);
-      setDeletedFromIndex(null);
+    const serverHasUserBlock =
+      blockAtInsert?.message.role === 'user' &&
+      blockAtInsert?.message.content === optimistic.userQuery;
+
+    if (!serverHasUserBlock) {
       return undefined;
     }
 
-    const currentSignature = JSON.stringify(
-      (apiData?.session?.blocks || []).map(b => [
-        b.id,
-        b.message.role,
-        b.message.content,
-        !!b.loading,
-      ])
+    const hasAssistantResponse = serverBlocks.some(
+      (b, i) => i > optimistic.insertIndex && b.message.role === 'assistant'
     );
 
-    if (currentSignature !== optimistic.baselineSignature) {
-      const serverBlocks = apiData?.session?.blocks || [];
-      const hasAssistantResponse = serverBlocks.some(
-        (b, i) => i >= optimistic.insertIndex && b.message.role === 'assistant'
-      );
-
-      if (hasAssistantResponse) {
-        setOptimistic(null);
-        setDeletedFromIndex(null);
-        return undefined;
-      }
-    }
-
-    // Always schedule the safety timeout so the optimistic UI cannot persist
-    // indefinitely when the server data signature never diverges from baseline.
-    const remaining = 30_000 - (Date.now() - optimistic.createdAt);
-    const timer = setTimeout(() => {
+    if (hasAssistantResponse) {
       setOptimistic(null);
       setDeletedFromIndex(null);
-    }, remaining);
-    return () => clearTimeout(timer);
+    }
+
+    return undefined;
   }, [apiData?.session?.blocks, optimistic]);
 
   // Detect PR creation errors and show error messages