Fix disappearing issue comments (#4557)

## Thinking Path

> - Paperclip is a control plane for AI-agent companies, so issue detail
pages are a primary surface for understanding agent work and human
feedback.
> - The relevant subsystem here is the issue comments/chat experience
across the React issue detail page and the server comment pagination
API.
> - Long issue threads were only surfacing the newest page of comments
at first render, which hid earlier human and agent messages behind extra
pagination.
> - The first UI fix exposed that the descending cursor path on the
server could also fail for older-page fetches, leaving the chat tab
stuck on an infinite "Loading earlier comments..." state.
> - This needed to be addressed in both layers so the chat tab can
surface earlier conversation history without manual recovery and without
server errors.
> - This pull request auto-loads earlier comment pages in the issue
detail chat view and fixes the descending cursor predicate used by issue
comment pagination.
> - The benefit is that long-running issues like `PAPA-103` now show the
missing conversation history near the top of the chat surface instead of
hiding it or failing to load it.

## What Changed

- Auto-load earlier issue comment pages in the issue detail chat tab
until the thread reaches a 150-comment cap or there are no older
comments left.
- Add UI-side guard logic and regression coverage for optimistic issue
comment pagination so the autoload behavior stops cleanly.
- Replace the raw SQL descending cursor predicate in
`issueService.listComments` with typed Drizzle comparisons for the
`(createdAt, id)` anchor tuple.
- Add a server regression test that paginates earlier comments in
descending order from an anchor comment.
- Smoke-test the exact previously failing seeded `PAPA-103` cursor path
on the isolated dev instance used for review.

## Verification

- `pnpm --filter @paperclipai/server exec vitest run
src/__tests__/issues-service.test.ts`
- `pnpm --filter @paperclipai/server typecheck`
- Manual smoke against seeded `PAPA-103` data on the isolated dev
server:
- `GET /api/issues/PAPA-103/comments?order=desc&limit=50` returns `200`
- `GET
/api/issues/PAPA-103/comments?after=765d3609-edc6-4d11-a8fe-d466affbe85d&order=desc&limit=50`
now returns `200` with 50 comments instead of `500`

## Risks

- Moderate UI/perf risk on very large threads because the chat tab now
prefetches multiple earlier pages on mount; the cap is intentionally
limited to 150 comments to bound that work.
- Low API risk because the server fix only changes the cursor predicate
construction for anchor-based comment pagination, but any mistake there
would affect older-comment paging order.

> I checked `ROADMAP.md` before opening this PR and this bug fix does
not duplicate planned core work.

## Model Used

- OpenAI Codex coding agent in the Paperclip local adapter environment.
The exact backend model ID and context window were not exposed
in-session. Tool-assisted workflow included shell execution, git/GitHub
CLI, local test execution, and targeted code edits.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [ ] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

ui/src/lib/optimistic-issue-comments.test.ts

@@ -12,6 +12,7 @@ import {
   matchesIssueRef,
   mergeIssueComments,
   removeIssueCommentFromPages,
+  shouldAutoloadOlderIssueComments,
   takeOptimisticIssueComment,
   upsertIssueComment,
   upsertIssueCommentInPages,
@@ -233,6 +234,45 @@ describe("optimistic issue comments", () => {
     ).toBe("comment-1");
   });
 
+  it("autoloads older chat comments while the initial thread is still under the threshold", () => {
+    expect(
+      shouldAutoloadOlderIssueComments({
+        activeDetailTab: "chat",
+        hasOlderComments: true,
+        loadedCommentCount: 50,
+        initialPageLoading: false,
+        olderPageLoading: false,
+        autoLoadLimit: 150,
+      }),
+    ).toBe(true);
+  });
+
+  it("does not autoload older comments outside the chat tab", () => {
+    expect(
+      shouldAutoloadOlderIssueComments({
+        activeDetailTab: "activity",
+        hasOlderComments: true,
+        loadedCommentCount: 50,
+        initialPageLoading: false,
+        olderPageLoading: false,
+        autoLoadLimit: 150,
+      }),
+    ).toBe(false);
+  });
+
+  it("stops autoloading once the initial comment window reaches the cap", () => {
+    expect(
+      shouldAutoloadOlderIssueComments({
+        activeDetailTab: "chat",
+        hasOlderComments: true,
+        loadedCommentCount: 150,
+        initialPageLoading: false,
+        olderPageLoading: false,
+        autoLoadLimit: 150,
+      }),
+    ).toBe(false);
+  });
+
   it("upserts paged comments without dropping older pages", () => {
     const nextPages = upsertIssueCommentInPages(
       [

ui/src/lib/optimistic-issue-comments.ts

@@ -150,6 +150,21 @@ export function getNextIssueCommentPageParam(
   return lastPage[lastPage.length - 1]?.id;
 }
 
+export function shouldAutoloadOlderIssueComments(params: {
+  activeDetailTab: string;
+  hasOlderComments: boolean;
+  loadedCommentCount: number;
+  initialPageLoading: boolean;
+  olderPageLoading: boolean;
+  autoLoadLimit: number;
+}) {
+  if (params.activeDetailTab !== "chat") return false;
+  if (!params.hasOlderComments) return false;
+  if (params.initialPageLoading || params.olderPageLoading) return false;
+  if (params.loadedCommentCount === 0) return false;
+  return params.loadedCommentCount < params.autoLoadLimit;
+}
+
 export function upsertIssueComment(
   comments: IssueComment[] | undefined,
   nextComment: IssueComment,

ui/src/pages/IssueDetail.tsx

@@ -51,6 +51,7 @@ import {
   matchesIssueRef,
   mergeIssueComments,
   removeIssueCommentFromPages,
+  shouldAutoloadOlderIssueComments,
   takeOptimisticIssueComment,
   upsertIssueCommentInPages,
   type IssueCommentReassignment,
@@ -152,6 +153,7 @@ type IssueDetailComment = (IssueComment | OptimisticIssueComment) & {
 
 const FEEDBACK_TERMS_URL = import.meta.env.VITE_FEEDBACK_TERMS_URL?.trim() || "https://paperclip.ing/tos";
 const ISSUE_COMMENT_PAGE_SIZE = 50;
+const ISSUE_COMMENT_AUTOLOAD_LIMIT = ISSUE_COMMENT_PAGE_SIZE * 3;
 const TREE_CONTROL_MODE_LABEL: Record<IssueTreeControlMode, string> = {
   pause: "Pause subtree",
   resume: "Resume subtree",
@@ -1103,6 +1105,18 @@ export function IssueDetail() {
     () => flattenIssueCommentPages(commentPages?.pages),
     [commentPages?.pages],
   );
+  const shouldPrefetchOlderComments = useMemo(
+    () =>
+      shouldAutoloadOlderIssueComments({
+        activeDetailTab: detailTab,
+        hasOlderComments: hasOlderComments ?? false,
+        loadedCommentCount: comments.length,
+        initialPageLoading: commentsLoading,
+        olderPageLoading: commentsLoadingOlder,
+        autoLoadLimit: ISSUE_COMMENT_AUTOLOAD_LIMIT,
+      }),
+    [comments.length, commentsLoading, commentsLoadingOlder, detailTab, hasOlderComments],
+  );
   const { data: interactions = [] } = useQuery({
     queryKey: queryKeys.issues.interactions(issueId!),
     queryFn: () => issuesApi.listInteractions(issueId!),
@@ -2537,6 +2551,10 @@ export function IssueDetail() {
   const loadOlderComments = useCallback(() => {
     void fetchOlderComments();
   }, [fetchOlderComments]);
+  useEffect(() => {
+    if (!shouldPrefetchOlderComments) return;
+    void fetchOlderComments();
+  }, [fetchOlderComments, shouldPrefetchOlderComments]);
   const handleCommentVote = useCallback(async (commentId: string, vote: "up" | "down", options?: { allowSharing?: boolean; reason?: string }) => {
     await feedbackVoteMutation.mutateAsync({
       targetType: "issue_comment",