[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.test.ts

@@ -634,6 +634,71 @@ describe("inbox helpers", () => {
     ]);
   });
 
+  it("keeps nested grandchild issues visible in keyboard navigation", () => {
+    const parentIssue = makeIssue("parent", true);
+    parentIssue.lastActivityAt = new Date("2026-03-11T01:00:00.000Z");
+    const childIssue = makeIssue("child", true);
+    childIssue.parentId = parentIssue.id;
+    childIssue.lastActivityAt = new Date("2026-03-11T02:00:00.000Z");
+    const grandchildIssue = makeIssue("grandchild", false);
+    grandchildIssue.parentId = childIssue.id;
+    grandchildIssue.lastActivityAt = new Date("2026-03-11T05:00:00.000Z");
+
+    const [section] = buildGroupedInboxSections(
+      getInboxWorkItems({ issues: [parentIssue, childIssue, grandchildIssue], approvals: [] }),
+      "none",
+      {},
+      { nestingEnabled: true },
+    );
+
+    expect(section?.displayItems.map((item) => item.kind === "issue" ? item.issue.id : "other")).toEqual([
+      parentIssue.id,
+    ]);
+    expect(section?.displayItems[0]?.timestamp).toBe(new Date("2026-03-11T05:00:00.000Z").getTime());
+
+    expect(
+      buildInboxKeyboardNavEntries([section!], new Set(), new Set()).map((entry) => entry.type === "top"
+        ? entry.item.kind === "issue" ? entry.item.issue.id : "other"
+        : entry.type === "child"
+          ? entry.issueId
+          : entry.groupKey),
+    ).toEqual([parentIssue.id, childIssue.id, grandchildIssue.id]);
+
+    expect(
+      buildInboxKeyboardNavEntries([section!], new Set(), new Set([childIssue.id])).map((entry) => entry.type === "top"
+        ? entry.item.kind === "issue" ? entry.item.issue.id : "other"
+        : entry.type === "child"
+          ? entry.issueId
+          : entry.groupKey),
+    ).toEqual([parentIssue.id, childIssue.id]);
+  });
+
+  it("stops cyclic child issue traversal when building keyboard navigation", () => {
+    const parentIssue = makeIssue("parent", true);
+    const childIssue = makeIssue("child", true);
+    childIssue.parentId = parentIssue.id;
+    parentIssue.parentId = childIssue.id;
+
+    const groupedSections = [
+      {
+        key: "workspace:default",
+        displayItems: [{ kind: "issue", timestamp: 2, issue: parentIssue } satisfies InboxWorkItem],
+        childrenByIssueId: new Map([
+          [parentIssue.id, [childIssue]],
+          [childIssue.id, [parentIssue]],
+        ]),
+      },
+    ];
+
+    expect(
+      buildInboxKeyboardNavEntries(groupedSections, new Set(), new Set()).map((entry) => entry.type === "top"
+        ? entry.item.kind === "issue" ? entry.item.issue.id : "other"
+        : entry.type === "child"
+          ? entry.issueId
+          : entry.groupKey),
+    ).toEqual([parentIssue.id, childIssue.id]);
+  });
+
   it("emits a group nav entry for labeled groups and omits children when the group is collapsed", () => {
     const visibleIssue = makeIssue("visible", true);
     const hiddenIssue = makeIssue("hidden", true);

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]));
     }
   }