feat(canvas): show loading state while agent history loads

Track agent history loading in the workspace and render a spinner with a
localized "loading conversation history" message in the assistant feed,
so the empty state no longer flashes before history resolves.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en-US.json

@@ -341,6 +341,7 @@
   "conversation": "Chat",
   "newChat": "New chat",
   "conversationHistory": "History",
+  "loadingConversationHistory": "Loading conversation history...",
   "scrollToLatest": "Scroll to latest",
   "searchConversationPlaceholder": "Search conversations",
   "deleteConversation": "Delete conversation",

frontend/src/i18n/locales/zh-CN.json

@@ -341,6 +341,7 @@
   "conversation": "对话",
   "newChat": "新对话",
   "conversationHistory": "历史对话",
+  "loadingConversationHistory": "正在加载历史对话...",
   "scrollToLatest": "回到底部",
   "searchConversationPlaceholder": "请输入搜索关键词",
   "deleteConversation": "删除对话",

frontend/src/ui/pages/CanvasWorkspace/index.tsx

@@ -575,6 +575,7 @@ export function CanvasWorkspace({ projectId, shareAccess = ownerWorkspaceAccess
   const { project, setProject, projectRef, viewport, setViewport, viewportRef, nodes, setNodes, nodesRef, applyProjectDocument } = useCanvasDocument();
   const { selectedId, selectedIds, setSelectedIds, selectedNodes, selectedNode, selectOnlyNode, selectNodes, toggleNodeSelection } = useCanvasSelection(nodes);
   const [messages, setMessages] = useState<AgentMessage[]>([]);
+  const [agentHistoryLoading, setAgentHistoryLoading] = useState(canViewChat);
   const [prompt, setPrompt] = useState("");
   const [agentStarterConfig, setAgentStarterConfig] = useState<AgentStarterConfig>(() => defaultAgentStarterConfig(t));
   const [agentThreads, setAgentThreads] = useState<AgentThreadSummary[]>([]);
@@ -901,17 +902,20 @@ export function CanvasWorkspace({ projectId, shareAccess = ownerWorkspaceAccess
   useEffect(() => {
     selectedAgentThreadIdRef.current = null;
     setSelectedAgentThreadId(null);
+    setAgentHistoryLoading(canViewChat);
     setAgentThreads([]);
     setAgentHistorySearch("");
     setAgentHistoryOpen(false);
     autoScrollAgentFeedRef.current = false;
-  }, [projectId]);
+  }, [canViewChat, projectId]);
 
   useEffect(() => {
     if (agentHistoryOpen) void loadAgentThreads();
   }, [agentHistoryOpen, loadAgentThreads]);
 
   useEffect(() => {
+    let active = true;
+    setAgentHistoryLoading(canViewChat);
     designGateway
       .getProject(projectId)
       .then(({ project: nextProject }) => {
@@ -961,7 +965,13 @@ export function CanvasWorkspace({ projectId, shareAccess = ownerWorkspaceAccess
           return;
         }
         setError(toUserMessageText(err, "generateError"));
+      })
+      .finally(() => {
+        if (active) setAgentHistoryLoading(false);
       });
+    return () => {
+      active = false;
+    };
   }, [applyProject, canViewChat, projectId, startProjectEvents]);
 
   useEffect(() => {
@@ -2649,6 +2659,7 @@ export function CanvasWorkspace({ projectId, shareAccess = ownerWorkspaceAccess
     autoScrollAgentFeedRef.current = false;
     selectedAgentThreadIdRef.current = newAgentConversationId;
     setSelectedAgentThreadId(newAgentConversationId);
+    setAgentHistoryLoading(false);
     setAgentHistoryOpen(false);
     setPrompt("");
     setMessages([]);
@@ -4124,7 +4135,12 @@ export function CanvasWorkspace({ projectId, shareAccess = ownerWorkspaceAccess
           </div>
 
           <div className="assistant-feed" ref={assistantFeedRef} onScroll={updateAssistantScrollButton}>
-            {!hasCurrentConversation && !agentTimelineActive ? (
+            {agentHistoryLoading ? (
+              <div className="assistant-history-loading" role="status" aria-live="polite" data-testid="agent-history-loading">
+                <Loader2 className="spin" size={20} aria-hidden="true" />
+                <span>{t("loadingConversationHistory")}</span>
+              </div>
+            ) : !hasCurrentConversation && !agentTimelineActive ? (
               <CanvasAgentEmptyState config={agentStarterConfig} disabled={!canEdit} onSelectPrompt={selectAgentStarterPrompt} />
             ) : (
               <CanvasAgentTimeline messages={messages} nodes={nodes} active={agentTimelineActive} />

frontend/src/ui/styles.css

@@ -3789,6 +3789,24 @@ button:disabled {
   background: #dfe0e2;
 }
 
+.assistant-history-loading {
+  width: 100%;
+  min-height: 100%;
+  flex: 1 0 auto;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 10px;
+  color: #8a9099;
+  font-size: 13px;
+  line-height: 20px;
+}
+
+.assistant-history-loading svg {
+  color: #646a73;
+}
+
 .assistant-scroll-to-end {
   position: absolute;
   inset-inline: 0;