[codex] Polish inbox nested issue UI (#4959)

## Thinking Path

> - Paperclip orchestrates AI agents through issue lists and
issue-thread interactions
> - The inbox must preserve nested issue visibility and keyboard
navigation as work decomposes into deeper sub-issues
> - Some UI polish issues made nested rows harder to scan and pending
question cancellation less covered
> - The issue list also had a small test indentation regression around
load-more behavior
> - This pull request tightens nested inbox rendering and related
issue-thread/list polish
> - The benefit is a more reliable operator inbox for multi-level work
trees

## What Changed

- Included nested grandchild issues in inbox keyboard navigation and
recursive row rendering.
- Sort parent rows by descendant activity so active subtrees remain
visible.
- Removed extra inbox card background styling in favor of the page
surface.
- Added regression coverage for pending question cancellation.
- Cleaned up the issue-list load-more test indentation.

## Verification

- `pnpm exec vitest run ui/src/lib/inbox.test.ts
ui/src/components/IssueChatThread.test.tsx
ui/src/components/IssuesList.test.tsx`
- Screenshots were not captured in this PR split; the visible flow is
covered by focused component/helper tests and should get browser QA in
the follow-up issue.

## Risks

- Medium risk: nested inbox rendering and keyboard navigation are
user-visible. The changes are localized to inbox grouping/rendering
helpers and covered by targeted tests.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool use and local command
execution. Exact context window was not exposed in the runtime.

## 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
- [x] 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

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/lib/inbox.ts

@@ -906,9 +906,26 @@ export function buildInboxNesting(items: InboxWorkItem[]): {
     }
   }
 
-  // Sort each child list by most recent activity
+  const subtreeActivityTimestamp = (issue: Issue, seen: ReadonlySet<string> = new Set()): number => {
+    const ownTimestamp = issueLastActivityTimestamp(issue);
+    if (seen.has(issue.id)) return ownTimestamp;
+    const nextSeen = new Set(seen);
+    nextSeen.add(issue.id);
+    const children = childrenByIssueId.get(issue.id) ?? [];
+    if (children.length === 0) return ownTimestamp;
+    return Math.max(
+      ownTimestamp,
+      ...children.map((child) => subtreeActivityTimestamp(child, nextSeen)),
+    );
+  };
+
+  // Sort each child list by most recent descendant activity, not just direct issue activity.
   for (const children of childrenByIssueId.values()) {
-    children.sort(sortIssuesByMostRecentActivity);
+    children.sort((a, b) => {
+      const activityDiff = subtreeActivityTimestamp(b) - subtreeActivityTimestamp(a);
+      if (activityDiff !== 0) return activityDiff;
+      return sortIssuesByMostRecentActivity(a, b);
+    });
   }
 
   // Build root issue items with group-adjusted timestamps
@@ -917,7 +934,7 @@ export function buildInboxNesting(items: InboxWorkItem[]): {
     .map((item) => {
       const children = childrenByIssueId.get(item.issue.id);
       if (!children?.length) return item;
-      const maxChildTs = Math.max(...children.map(issueLastActivityTimestamp));
+      const maxChildTs = Math.max(...children.map((child) => subtreeActivityTimestamp(child)));
       return { ...item, timestamp: Math.max(item.timestamp, maxChildTs) };
     });
 
@@ -985,6 +1002,23 @@ export function buildInboxKeyboardNavEntries(
     }
     if (isCollapsed) continue;
 
+    const addIssueChildren = (issueId: string, seen: ReadonlySet<string>) => {
+      const children = group.childrenByIssueId.get(issueId);
+      if (!children?.length || collapsedInboxParents.has(issueId)) return;
+
+      for (const child of children) {
+        if (seen.has(child.id)) continue;
+        const nextSeen = new Set(seen);
+        nextSeen.add(child.id);
+        entries.push({
+          type: "child",
+          issueId: child.id,
+          issue: child,
+        });
+        addIssueChildren(child.id, nextSeen);
+      }
+    };
+
     for (const item of group.displayItems) {
       entries.push({
         type: "top",
@@ -993,17 +1027,7 @@ export function buildInboxKeyboardNavEntries(
       });
 
       if (item.kind !== "issue") continue;
-
-      const children = group.childrenByIssueId.get(item.issue.id);
-      if (!children?.length || collapsedInboxParents.has(item.issue.id)) continue;
-
-      for (const child of children) {
-        entries.push({
-          type: "child",
-          issueId: child.id,
-          issue: child,
-        });
-      }
+      addIssueChildren(item.issue.id, new Set([item.issue.id]));
     }
   }