[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-messages.ts

@@ -81,6 +81,11 @@ type MessageWithOrder = {
   message: ThreadMessage;
 };
 
+export interface StableThreadMessageCacheEntry {
+  fingerprint: string;
+  message: ThreadMessage;
+}
+
 function toDate(value: Date | string | null | undefined) {
   return value instanceof Date ? value : new Date(value ?? Date.now());
 }
@@ -89,6 +94,41 @@ function toTimestamp(value: Date | string | null | undefined) {
   return toDate(value).getTime();
 }
 
+function fingerprintThreadMessage(message: ThreadMessage) {
+  return JSON.stringify(message);
+}
+
+export function stabilizeThreadMessages(
+  messages: readonly ThreadMessage[],
+  previousMessages: readonly ThreadMessage[],
+  previousById: ReadonlyMap<string, StableThreadMessageCacheEntry>,
+) {
+  const nextById = new Map<string, StableThreadMessageCacheEntry>();
+  let sameSequence = previousMessages.length === messages.length;
+
+  const stabilizedMessages = messages.map((message, index) => {
+    const fingerprint = fingerprintThreadMessage(message);
+    const cached = previousById.get(message.id);
+    const stableMessage =
+      cached && cached.fingerprint === fingerprint
+        ? cached.message
+        : message;
+    nextById.set(message.id, {
+      fingerprint,
+      message: stableMessage,
+    });
+    if (sameSequence && previousMessages[index] !== stableMessage) {
+      sameSequence = false;
+    }
+    return stableMessage;
+  });
+
+  return {
+    messages: sameSequence ? previousMessages : stabilizedMessages,
+    cache: nextById,
+  };
+}
+
 function sortByCreated<T extends { createdAt: Date | string; id: string }>(items: readonly T[]) {
   return [...items].sort((a, b) => {
     const diff = toTimestamp(a.createdAt) - toTimestamp(b.createdAt);