🤖 Fix workspace recency after compaction

When chat history is compacted, user messages are replaced with a summary
(assistant message with compacted=true). The recency calculation only checked
for user messages, causing compacted workspaces to sink to the bottom.

Changes:
- Rename WorkspaceState.lastUserMessageAt → recencyTimestamp (clearer semantics)
- Add fallback: if no user messages, use most recent compacted message timestamp
- Keeps Date.now() for new compaction summaries (workspace jumps to top, correct UX)

This ensures workspaces with only compacted history maintain their position
in the sidebar, while still preferring user messages during active streams.

Generated with `cmux`

Author: ammar-agent

src/hooks/useWorkspaceAggregators.ts

@@ -31,7 +31,7 @@ export interface WorkspaceState {
   loading: boolean;
   cmuxMessages: CmuxMessage[];
   currentModel: string;
-  lastUserMessageAt: number | null; // Timestamp of most recent user message (null if no user messages)
+  recencyTimestamp: number | null; // Timestamp for sorting: most recent user message, or compacted message if no user messages
 }
 
 /**
@@ -69,14 +69,27 @@ export function useWorkspaceAggregators(workspaceMetadata: Map<string, Workspace
       const isCaughtUp = caughtUpRef.current.get(workspaceId) ?? false;
       const activeStreams = aggregator.getActiveStreams();
 
-      // Get most recent user message timestamp (persisted, survives restarts)
-      // Using user messages instead of assistant messages avoids constant reordering
-      // when multiple concurrent streams are running
+      // Get recency timestamp for sorting:
+      // 1. Prefer most recent user message (avoids constant reordering during concurrent streams)
+      // 2. Fallback to most recent compacted message (prevents workspace from dropping to bottom after compaction)
+      // 3. null if no messages with timestamps
       const messages = aggregator.getAllMessages();
       const lastUserMsg = [...messages]
         .reverse()
         .find((m) => m.role === "user" && m.metadata?.timestamp);
-      const lastUserMessageAt = lastUserMsg?.metadata?.timestamp ?? null;
+      
+      let recencyTimestamp: number | null = null;
+      if (lastUserMsg?.metadata?.timestamp) {
+        recencyTimestamp = lastUserMsg.metadata.timestamp;
+      } else {
+        // No user messages - check for compacted messages
+        const lastCompactedMsg = [...messages]
+          .reverse()
+          .find((m) => m.metadata?.compacted === true && m.metadata?.timestamp);
+        if (lastCompactedMsg?.metadata?.timestamp) {
+          recencyTimestamp = lastCompactedMsg.metadata.timestamp;
+        }
+      }
 
       return {
         messages: aggregator.getDisplayedMessages(),
@@ -85,7 +98,7 @@ export function useWorkspaceAggregators(workspaceMetadata: Map<string, Workspace
         loading: !hasMessages && !isCaughtUp,
         cmuxMessages: aggregator.getAllMessages(),
         currentModel: aggregator.getCurrentModel() ?? "claude-sonnet-4-5",
-        lastUserMessageAt,
+        recencyTimestamp,
       };
     },
     [getAggregator]
@@ -343,8 +356,8 @@ export function useWorkspaceAggregators(workspaceMetadata: Map<string, Workspace
     () => {
       const timestamps: Record<string, number> = {};
       for (const [id, state] of workspaceStates) {
-        if (state.lastUserMessageAt !== null) {
-          timestamps[id] = state.lastUserMessageAt;
+        if (state.recencyTimestamp !== null) {
+          timestamps[id] = state.recencyTimestamp;
         }
       }
       return timestamps;