fix(webui): preserve range selection and turn timing

webui/src/components/thread/ThreadMessages.tsx

@@ -76,8 +76,10 @@ export function ThreadMessages({
   );
   const forkFlags = useMemo(() => assistantForkFlags(units), [units]);
   const liveActivityClusterIndices = useMemo(
-    () => isStreaming ? currentActivityClusterIndices(units) : new Set<number>(),
-    [isStreaming, units],
+    () => isStreaming
+      ? currentActivityClusterIndices(units, activeTurnId)
+      : new Set<number>(),
+    [activeTurnId, isStreaming, units],
   );
   const pendingTurn = useMemo(
     () => pendingTurnProjection(messages, activeTurnId),
@@ -362,8 +364,24 @@ function ForkBoundaryDivider({ label }: { label: string }) {
   );
 }
 
-function currentActivityClusterIndices(units: DisplayUnit[]): Set<number> {
+function currentActivityClusterIndices(
+  units: DisplayUnit[],
+  activeTurnId: string | null,
+): Set<number> {
   const indices = new Set<number>();
+  if (activeTurnId) {
+    for (let i = units.length - 1; i >= 0; i -= 1) {
+      const unit = units[i];
+      if (
+        unit.type === "activity"
+        && unit.messages.some((message) => message.turnId === activeTurnId)
+      ) {
+        indices.add(i);
+        return indices;
+      }
+    }
+  }
+
   let markedCurrentActivity = false;
   for (let i = units.length - 1; i >= 0; i -= 1) {
     const unit = units[i];

webui/src/components/thread/ThreadShell.tsx

@@ -814,8 +814,11 @@ export function ThreadShell({
   const rememberedViewportTurnId = chatId
     ? activeViewportTurnByChatIdRef.current.get(chatId) ?? null
     : null;
+  const canonicalRunTurnId = chatId && messagesReady && turnActive
+    ? client.getRunTurnId(chatId)
+    : null;
   const viewportTurnId = messagesReady && turnActive
-    ? rememberedViewportTurnId ?? restoredViewportTurnId
+    ? canonicalRunTurnId ?? rememberedViewportTurnId ?? restoredViewportTurnId
     : null;
   const activeTurnStartedHere =
     viewportTurnId !== null && viewportTurnId === submittedViewportTurnId;
@@ -1313,7 +1316,12 @@ export function ThreadShell({
     (content: string, images?: SendAttachment[], options?: SendOptions) => {
       setFallbackModelName(null);
       const submitted = send(content, images, withWorkspaceScope(options));
-      if (chatId && submitted && !submitted.sideChannel) {
+      if (
+        chatId
+        && submitted
+        && !submitted.sideChannel
+        && options?.continueActiveTurn !== true
+      ) {
         activeViewportTurnByChatIdRef.current.set(chatId, submitted.turnId);
         setSubmittedViewportTurnId(submitted.turnId);
       }