Fix live issue chat segment activity

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/IssueChatThread.tsx

@@ -48,6 +48,7 @@ import { usePaperclipIssueRuntime, type PaperclipIssueRuntimeReassignment } from
 import {
   buildIssueChatMessages,
   formatDurationWords,
+  isCoTSegmentActive,
   stabilizeThreadMessages,
   type IssueChatComment,
   type IssueChatLinkedRun,
@@ -851,13 +852,16 @@ function IssueChatChainOfThought({
     (p): p is ToolCallMessagePart => p.type === "tool-call",
   );
 
-  const isActive = isMessageRunning;
-  const [expanded, setExpanded] = useState(isActive);
-
   const rawSegments = Array.isArray(custom.chainOfThoughtSegments)
     ? (custom.chainOfThoughtSegments as SegmentTiming[])
     : [];
   const segmentTiming = myIndex >= 0 ? rawSegments[myIndex] ?? null : null;
+  const isActive = isCoTSegmentActive({
+    isMessageRunning,
+    segmentIndex: myIndex,
+    segmentCount: rawSegments.length,
+  });
+  const [expanded, setExpanded] = useState(isActive);
   const liveElapsed = useLiveElapsed(segmentTiming?.startMs, isActive);
 
   useEffect(() => {

ui/src/components/IssueChatThreadClassic.tsx

@@ -59,6 +59,7 @@ import { usePaperclipIssueRuntime, type PaperclipIssueRuntimeReassignment } from
 import {
   buildIssueChatMessages,
   formatDurationWords,
+  isCoTSegmentActive,
   stabilizeThreadMessages,
   type IssueChatComment,
   type IssueChatLinkedRun,
@@ -876,13 +877,16 @@ function IssueChatChainOfThought({
     (p): p is ToolCallMessagePart => p.type === "tool-call",
   );
 
-  const isActive = isMessageRunning;
-  const [expanded, setExpanded] = useState(isActive);
-
   const rawSegments = Array.isArray(custom.chainOfThoughtSegments)
     ? (custom.chainOfThoughtSegments as SegmentTiming[])
     : [];
   const segmentTiming = myIndex >= 0 ? rawSegments[myIndex] ?? null : null;
+  const isActive = isCoTSegmentActive({
+    isMessageRunning,
+    segmentIndex: myIndex,
+    segmentCount: rawSegments.length,
+  });
+  const [expanded, setExpanded] = useState(isActive);
   const liveElapsed = useLiveElapsed(segmentTiming?.startMs, isActive);
 
   useEffect(() => {

ui/src/lib/issue-chat-messages.test.ts

@@ -3,6 +3,7 @@ import type { Agent } from "@paperclipai/shared";
 import {
   buildAssistantPartsFromTranscript,
   buildIssueChatMessages,
+  isCoTSegmentActive,
   stabilizeThreadMessages,
   type IssueChatComment,
   type IssueChatLinkedRun,
@@ -238,6 +239,24 @@ describe("buildAssistantPartsFromTranscript", () => {
     }]);
   });
 
+  it("marks only the latest chain-of-thought segment active while a run is live", () => {
+    expect(isCoTSegmentActive({
+      isMessageRunning: true,
+      segmentIndex: 0,
+      segmentCount: 2,
+    })).toBe(false);
+    expect(isCoTSegmentActive({
+      isMessageRunning: true,
+      segmentIndex: 1,
+      segmentCount: 2,
+    })).toBe(true);
+    expect(isCoTSegmentActive({
+      isMessageRunning: false,
+      segmentIndex: 1,
+      segmentCount: 2,
+    })).toBe(false);
+  });
+
   it("keeps run errors while suppressing init and system transcript noise", () => {
     const result = buildAssistantPartsFromTranscript([
       {

ui/src/lib/issue-chat-messages.ts

@@ -538,6 +538,17 @@ export interface SegmentTiming {
   endMs: number;
 }
 
+export function isCoTSegmentActive(args: {
+  isMessageRunning: boolean;
+  segmentIndex: number;
+  segmentCount: number;
+}) {
+  const { isMessageRunning, segmentIndex, segmentCount } = args;
+  if (!isMessageRunning) return false;
+  if (segmentCount <= 0 || segmentIndex < 0) return true;
+  return segmentIndex === segmentCount - 1;
+}
+
 function computeSegmentTimings(entries: readonly IssueChatTranscriptEntry[]): SegmentTiming[] {
   const timings: SegmentTiming[] = [];
   let inSegment = false;