test(ui): reply-to-comment scenario in the Issue Chat UX Lab

Adds a fixture-backed reply section to /tests/ux/chat exercising the P3 states:
composer reply chip (seeded via localStorage), sent quoted block, and the
deleted-source tombstone. Gives QA a live in-app surface to test against.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ui/src/fixtures/issueChatUxFixtures.ts

@@ -338,6 +338,75 @@ export const issueChatUxReviewEvents: IssueTimelineEvent[] = [
   },
 ];
 
+/**
+ * Reply-to-comment (PAP-13802 P3) scenario: a sent reply with a quoted block, a reply whose
+ * source was deleted (tombstone), and the source comments they point back to. Seed
+ * {@link issueChatUxReplyDraftKey} in localStorage to also render the composer reply chip.
+ */
+export const issueChatUxReplyDraftKey = "paperclip:issue-comment-draft:issue-ux-reply";
+
+export const issueChatUxReplyComments: IssueChatComment[] = [
+  createComment({
+    id: "comment-reply-source",
+    authorAgentId: primaryAgent.id,
+    authorUserId: null,
+    body: "I moved the composer reply chip above the editor and clamp the quoted source to 200 chars so long comments stay one line.",
+    createdAt: new Date("2026-04-06T13:00:00.000Z"),
+    updatedAt: new Date("2026-04-06T13:00:00.000Z"),
+    runId: "run-reply-1",
+    runAgentId: primaryAgent.id,
+  }),
+  createComment({
+    id: "comment-reply-sent",
+    body: "Nice — can you make the quoted block jump to the original when clicked?",
+    createdAt: new Date("2026-04-06T13:02:00.000Z"),
+    updatedAt: new Date("2026-04-06T13:02:00.000Z"),
+    metadata: {
+      version: 1,
+      sections: [],
+      replyTo: {
+        commentId: "comment-reply-source",
+        authorType: "agent",
+        authorAgentId: primaryAgent.id,
+        authorUserId: null,
+        excerpt: "I moved the composer reply chip above the editor and clamp the quoted source to 200 chars so long comments stay one line.",
+        excerptTruncated: true,
+      },
+    },
+  }),
+  createComment({
+    id: "comment-reply-deleted-source",
+    body: "This source comment was removed after the reply was posted.",
+    createdAt: new Date("2026-04-06T13:04:00.000Z"),
+    updatedAt: new Date("2026-04-06T13:04:30.000Z"),
+    deletedAt: new Date("2026-04-06T13:05:00.000Z"),
+    deletedByType: "user",
+    deletedByUserId: "user-1",
+  }),
+  createComment({
+    id: "comment-reply-tombstone",
+    authorAgentId: reviewAgent.id,
+    authorUserId: null,
+    body: "Done — and when the source is deleted the quoted header falls back to a tombstone.",
+    createdAt: new Date("2026-04-06T13:06:00.000Z"),
+    updatedAt: new Date("2026-04-06T13:06:00.000Z"),
+    runId: "run-reply-2",
+    runAgentId: reviewAgent.id,
+    metadata: {
+      version: 1,
+      sections: [],
+      replyTo: {
+        commentId: "comment-reply-deleted-source",
+        authorType: "user",
+        authorAgentId: null,
+        authorUserId: "user-1",
+        excerpt: "This source comment was removed after the reply was posted.",
+        excerptTruncated: false,
+      },
+    },
+  }),
+];
+
 export const issueChatUxFeedbackVotes: FeedbackVote[] = [
   {
     id: "feedback-1",

ui/src/pages/IssueChatUxLab.tsx

@@ -16,6 +16,8 @@ import {
   issueChatUxLiveRuns,
   issueChatUxMentions,
   issueChatUxReassignOptions,
+  issueChatUxReplyComments,
+  issueChatUxReplyDraftKey,
   issueChatUxReviewComments,
   issueChatUxReviewEvents,
   issueChatUxSubmittingComments,
@@ -143,6 +145,24 @@ function RotatingReasoningDemo({ intervalMs = 2200 }: { intervalMs?: number }) {
 export function IssueChatUxLab() {
   const [showComposer, setShowComposer] = useState(true);
 
+  // Seed a composer reply draft so the reply-scenario section renders the chip on load.
+  useEffect(() => {
+    try {
+      localStorage.setItem(
+        `${issueChatUxReplyDraftKey}:reply`,
+        JSON.stringify({
+          commentId: "comment-reply-source",
+          authorName: "CodexCoder",
+          excerpt:
+            "I moved the composer reply chip above the editor and clamp the quoted source to 200 chars so long comments stay one line.",
+          excerptTruncated: true,
+        }),
+      );
+    } catch {
+      // Ignore localStorage failures in the lab preview.
+    }
+  }, []);
+
   return (
     <div className="space-y-6">
       <div className="overflow-hidden rounded-(--rad-32) border border-border/70 bg-[linear-gradient(135deg,rgba(8,145,178,0.10),transparent_28%),linear-gradient(180deg,rgba(245,158,11,0.10),transparent_44%),var(--background)] shadow-[0_30px_80px_rgba(15,23,42,0.10)]">
@@ -296,6 +316,29 @@ export function IssueChatUxLab() {
         />
       </LabSection>
 
+      <LabSection
+        id="reply-to-comment"
+        eyebrow="Reply to comment"
+        title="Reply chip, quoted block, and tombstone"
+        description="PAP-13802 P3: hover a bubble to reveal Reply, the composer shows a quoted/clamped source chip (click to jump to the original, X or Escape to clear), sent replies render a quoted header above the body, and a reply whose source was deleted falls back to an 'Original comment deleted' tombstone."
+        accentClassName="bg-[linear-gradient(180deg,rgba(16,185,129,0.06),transparent_28%),var(--background)]"
+      >
+        <IssueChatThread
+          comments={issueChatUxReplyComments}
+          linkedRuns={[]}
+          timelineEvents={[]}
+          issueStatus="in_progress"
+          agentMap={issueChatUxAgentMap}
+          currentUserId="user-1"
+          onAdd={noop}
+          onVote={noop}
+          draftKey={issueChatUxReplyDraftKey}
+          mentions={issueChatUxMentions}
+          showComposer={showComposer}
+          enableLiveTranscriptPolling={false}
+        />
+      </LabSection>
+
       <LabSection
         eyebrow="Submitting state"
         title="Pending message bubble"