feat(webui): polish sidebar and session transitions (#5393)

webui/src/components/thread/AgentActivityCluster.tsx

@@ -319,7 +319,7 @@ export function AgentActivityCluster({
     syncActivityScrollFade();
   }, [syncActivityScrollFade]);
 
-  if (!hasVisibleActivity) return null;
+  if (!hasVisibleActivity && !isTurnStreaming) return null;
 
   if (hasOnlyFileActivity) {
     return (
@@ -343,6 +343,7 @@ export function AgentActivityCluster({
         contentRef={activityContentRef}
         fadeTop={activityScrollFade.top}
         fadeBottom={activityScrollFade.bottom}
+        hasDetails={hasVisibleActivity}
         onToggle={toggleOuter}
         onScroll={onActivityScroll}
       >
@@ -382,7 +383,13 @@ function activityDurationMs(
   const timestamps = messages
     .map((message) => message.createdAt)
     .filter((value) => Number.isFinite(value));
-  if (!timestamps.length) return 0;
+  if (!timestamps.length) {
+    return active
+      && typeof activeStartedAtMs === "number"
+      && Number.isFinite(activeStartedAtMs)
+      ? Math.max(0, now - activeStartedAtMs)
+      : 0;
+  }
   const first = active && Number.isFinite(activeStartedAtMs)
     ? activeStartedAtMs!
     : Math.min(...timestamps);