[codex] Improve issue detail and issue-list UX (#3678)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - A core part of that is the operator experience around reading issue
state, agent chat, and sub-task structure
> - The current branch had a long run of issue-detail and issue-list UX
fixes that all improve how humans follow and steer active work
> - Those changes mostly live in the UI/chat surface and should be
reviewed together instead of mixed with workspace/runtime work
> - This pull request packages the issue-detail, chat, markdown, and
sub-issue list improvements into one standalone change
> - The benefit is a cleaner, less jumpy, more reliable issue workflow
on desktop and mobile without coupling it to unrelated server/runtime
refactors

## What Changed

- Stabilized issue chat runtime wiring, optimistic comment handling,
queued-comment cancellation, and composer anchoring during live updates
- Fixed several issue-detail rendering and navigation regressions
including placeholder bleed, local polling scope, mobile inbox-to-issue
transitions, and visible refresh resets
- Improved markdown and rich-content handling with advisory image
normalization, editor fallback behavior, touch mention recovery, and
`issue:` quicklook links
- Refined sub-issue behavior with parent-derived defaults, current-user
inheritance fixes, empty-state cleanup, and a reusable issue-list
presentation for sub-issues
- Added targeted UI tests for the new issue-detail, chat scroll/message,
placeholder-data, markdown, and issue-list behaviors

## Verification

- `pnpm vitest run ui/src/components/IssueChatThread.test.tsx
ui/src/components/MarkdownEditor.test.tsx
ui/src/components/IssuesList.test.tsx
ui/src/context/LiveUpdatesProvider.test.tsx
ui/src/lib/issue-chat-messages.test.ts
ui/src/lib/issue-chat-scroll.test.ts
ui/src/lib/issue-detail-subissues.test.ts
ui/src/lib/query-placeholder-data.test.tsx
ui/src/hooks/usePaperclipIssueRuntime.test.tsx`

## Risks

- Medium: this branch touches the highest-traffic issue-detail UI paths,
so regressions would show up as chat/thread or sub-issue UX glitches
- The changes are UI-heavy and would benefit from reviewer screenshots
or a quick manual browser pass before merge

## Model Used

- OpenAI Codex coding agent (GPT-5-class runtime in Codex CLI; exact
deployed model ID is not exposed in this environment), reasoning
enabled, tool use and local code execution enabled

## 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 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

---------

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

ui/src/lib/subIssueDefaults.test.ts

@@ -0,0 +1,136 @@
+import { describe, expect, it } from "vitest";
+import type { ExecutionWorkspace, Issue } from "@paperclipai/shared";
+import { buildSubIssueDefaults, buildSubIssueDefaultsForViewer } from "./subIssueDefaults";
+
+function makeExecutionWorkspace(overrides: Partial<ExecutionWorkspace> = {}): ExecutionWorkspace {
+  return {
+    id: "workspace-1",
+    companyId: "company-1",
+    projectId: "project-1",
+    projectWorkspaceId: "project-workspace-1",
+    sourceIssueId: null,
+    status: "active",
+    mode: "isolated_workspace",
+    strategyType: "git_worktree",
+    name: "Parent workspace",
+    cwd: "/tmp/workspace-1",
+    repoUrl: null,
+    baseRef: null,
+    branchName: "feature/pap-1",
+    providerType: "git_worktree",
+    providerRef: null,
+    derivedFromExecutionWorkspaceId: null,
+    openedAt: new Date("2026-04-07T00:00:00.000Z"),
+    closedAt: null,
+    cleanupEligibleAt: null,
+    cleanupReason: null,
+    config: null,
+    metadata: null,
+    lastUsedAt: new Date("2026-04-07T00:00:00.000Z"),
+    createdAt: new Date("2026-04-07T00:00:00.000Z"),
+    updatedAt: new Date("2026-04-07T00:00:00.000Z"),
+    ...overrides,
+  };
+}
+
+function makeIssue(overrides: Partial<Issue> = {}): Issue {
+  return {
+    id: "issue-1",
+    identifier: "PAP-1",
+    companyId: "company-1",
+    projectId: "project-1",
+    projectWorkspaceId: "project-workspace-1",
+    goalId: "goal-1",
+    parentId: null,
+    title: "Parent issue",
+    description: null,
+    status: "todo",
+    priority: "medium",
+    assigneeAgentId: null,
+    assigneeUserId: null,
+    checkoutRunId: null,
+    executionRunId: null,
+    executionAgentNameKey: null,
+    executionLockedAt: null,
+    createdByAgentId: null,
+    createdByUserId: null,
+    issueNumber: 1,
+    requestDepth: 0,
+    billingCode: null,
+    assigneeAdapterOverrides: null,
+    executionWorkspaceId: null,
+    executionWorkspacePreference: "shared_workspace",
+    executionWorkspaceSettings: null,
+    currentExecutionWorkspace: null,
+    startedAt: null,
+    completedAt: null,
+    cancelledAt: null,
+    hiddenAt: null,
+    createdAt: new Date("2026-04-07T00:00:00.000Z"),
+    updatedAt: new Date("2026-04-07T00:00:00.000Z"),
+    ...overrides,
+  };
+}
+
+describe("buildSubIssueDefaults", () => {
+  it("inherits the parent agent assignee and workspace context", () => {
+    const defaults = buildSubIssueDefaults(
+      makeIssue({
+        assigneeAgentId: "agent-1",
+        executionWorkspaceId: "workspace-1",
+        currentExecutionWorkspace: makeExecutionWorkspace(),
+      }),
+    );
+
+    expect(defaults).toEqual({
+      parentId: "issue-1",
+      parentIdentifier: "PAP-1",
+      parentTitle: "Parent issue",
+      projectId: "project-1",
+      projectWorkspaceId: "project-workspace-1",
+      goalId: "goal-1",
+      executionWorkspaceId: "workspace-1",
+      executionWorkspaceMode: "reuse_existing",
+      parentExecutionWorkspaceLabel: "Parent workspace",
+      assigneeAgentId: "agent-1",
+    });
+  });
+
+  it("inherits a user assignee when the parent is assigned to a user", () => {
+    const defaults = buildSubIssueDefaults(
+      makeIssue({
+        assigneeUserId: "user-1",
+      }),
+    );
+
+    expect(defaults).toEqual({
+      parentId: "issue-1",
+      parentIdentifier: "PAP-1",
+      parentTitle: "Parent issue",
+      projectId: "project-1",
+      projectWorkspaceId: "project-workspace-1",
+      goalId: "goal-1",
+      executionWorkspaceMode: "shared_workspace",
+      assigneeUserId: "user-1",
+    });
+  });
+
+  it("leaves the sub-issue unassigned when the parent assignee is the current user", () => {
+    const defaults = buildSubIssueDefaultsForViewer(
+      makeIssue({
+        assigneeUserId: "user-1",
+      }),
+      "user-1",
+    );
+
+    expect(defaults).toEqual({
+      parentId: "issue-1",
+      parentIdentifier: "PAP-1",
+      parentTitle: "Parent issue",
+      projectId: "project-1",
+      projectWorkspaceId: "project-workspace-1",
+      goalId: "goal-1",
+      executionWorkspaceMode: "shared_workspace",
+    });
+  });
+});