[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/issue-chat-scroll.test.ts

@@ -0,0 +1,98 @@
+// @vitest-environment jsdom
+
+import { describe, expect, it, vi } from "vitest";
+import {
+  captureComposerViewportSnapshot,
+  restoreComposerViewportSnapshot,
+  shouldPreserveComposerViewport,
+} from "./issue-chat-scroll";
+
+function mockTop(element: HTMLElement, top: number) {
+  vi.spyOn(element, "getBoundingClientRect").mockReturnValue({
+    top,
+    bottom: top + 48,
+    left: 0,
+    right: 0,
+    width: 0,
+    height: 48,
+    x: 0,
+    y: top,
+    toJSON: () => ({}),
+  } as DOMRect);
+}
+
+describe("issue-chat-scroll", () => {
+  it("restores page scroll when the composer shifts in the viewport", () => {
+    const composer = document.createElement("div");
+    document.body.appendChild(composer);
+    const scrollByMock = vi.spyOn(window, "scrollBy").mockImplementation(() => {});
+
+    mockTop(composer, 420);
+    const snapshot = captureComposerViewportSnapshot(composer);
+
+    mockTop(composer, 560);
+    restoreComposerViewportSnapshot(snapshot, composer);
+
+    expect(scrollByMock).toHaveBeenCalledWith({ top: 140, left: 0, behavior: "auto" });
+
+    scrollByMock.mockRestore();
+    composer.remove();
+  });
+
+  it("restores main-content scroll when the layout uses an internal scroller", () => {
+    const mainContent = document.createElement("main");
+    mainContent.id = "main-content";
+    mainContent.style.overflowY = "auto";
+    Object.defineProperty(mainContent, "scrollHeight", {
+      configurable: true,
+      value: 1800,
+    });
+    Object.defineProperty(mainContent, "clientHeight", {
+      configurable: true,
+      value: 900,
+    });
+    mainContent.scrollTop = 240;
+    document.body.appendChild(mainContent);
+
+    const composer = document.createElement("div");
+    document.body.appendChild(composer);
+    const scrollByMock = vi.spyOn(window, "scrollBy").mockImplementation(() => {});
+
+    mockTop(composer, 300);
+    const snapshot = captureComposerViewportSnapshot(composer);
+
+    mockTop(composer, 380);
+    restoreComposerViewportSnapshot(snapshot, composer);
+
+    expect(mainContent.scrollTop).toBe(320);
+    expect(scrollByMock).not.toHaveBeenCalled();
+
+    scrollByMock.mockRestore();
+    composer.remove();
+    mainContent.remove();
+  });
+
+  it("does not preserve the composer viewport just because the composer is visible", () => {
+    const composer = document.createElement("div");
+    document.body.appendChild(composer);
+    mockTop(composer, 540);
+
+    expect(shouldPreserveComposerViewport(composer)).toBe(false);
+
+    composer.remove();
+  });
+
+  it("preserves the composer viewport when focus stays inside the composer", () => {
+    const composer = document.createElement("div");
+    const input = document.createElement("textarea");
+    composer.appendChild(input);
+    document.body.appendChild(composer);
+    mockTop(composer, 1200);
+
+    input.focus();
+
+    expect(shouldPreserveComposerViewport(composer)).toBe(true);
+
+    composer.remove();
+  });
+});