Improve issue thread scale and markdown polish (#4861)

## Thinking Path

> - Paperclip's board UI is the operator surface for supervising
AI-agent companies.
> - Issue threads are where operators read progress, respond to agents,
inspect markdown, and jump through long histories.
> - Large threads and rich markdown had become difficult to navigate and
expensive to render.
> - The previous rollup mixed these UI scale fixes with unrelated
backend recovery, costs, backups, and settings changes.
> - This pull request isolates the issue-thread scale and markdown
polish work.
> - The benefit is a reviewable UI slice that can merge independently of
the backend reliability, database backup, workflow, and board QoL PRs.

## What Changed

- Virtualized long issue chat threads and stabilized
anchor/jump-to-latest behavior for large histories.
- Added incremental issue-list row loading and tests for
scroll-triggered pagination behavior.
- Hardened markdown body rendering and markdown editor behavior around
HTML tags, image drops, code-copy UI, and escaped newline handling.
- Added a long-thread measurement harness at
`scripts/measure-issue-chat-long-thread.mjs` plus
`perf:issue-chat-long-thread`.
- Added focused UI/lib regression coverage for thread rendering,
markdown, optimistic comments, and message building.

## Verification

- `pnpm install --frozen-lockfile`
- `pnpm exec vitest run ui/src/components/IssueChatThread.test.tsx
ui/src/components/IssuesList.test.tsx
ui/src/components/MarkdownBody.test.tsx
ui/src/components/MarkdownEditor.test.tsx
ui/src/lib/issue-chat-messages.test.ts
ui/src/lib/optimistic-issue-comments.test.ts`
- Result: 6 test files passed, 170 tests passed.
- UI screenshots not included because this PR is covered by targeted
component tests and does not introduce a new page layout.

## Risks

- Virtualization changes can affect scroll anchoring in edge cases on
very long threads.
- Markdown/editor hardening changes are intentionally defensive, but
malformed content may render differently than before.

> 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-5.5, code execution and GitHub CLI tool use, medium
reasoning effort.

## 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
- [x] 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/components/NewIssueDialog.tsx

@@ -417,6 +417,7 @@ export function NewIssueDialog() {
   const [isFileDragOver, setIsFileDragOver] = useState(false);
   const draftTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
   const executionWorkspaceDefaultProjectId = useRef<string | null>(null);
+  const initializationKeyRef = useRef<string | null>(null);
 
   const effectiveCompanyId = dialogCompanyId ?? selectedCompanyId;
   const dialogCompany = companies.find((c) => c.id === effectiveCompanyId) ?? selectedCompany;
@@ -673,7 +674,13 @@ export function NewIssueDialog() {
 
   // Restore draft or apply defaults when dialog opens
   useEffect(() => {
-    if (!newIssueOpen) return;
+    if (!newIssueOpen) {
+      initializationKeyRef.current = null;
+      return;
+    }
+    const initializationKey = `${selectedCompanyId ?? ""}:${JSON.stringify(newIssueDefaults)}`;
+    if (initializationKeyRef.current === initializationKey) return;
+    initializationKeyRef.current = initializationKey;
     setDialogCompanyId(selectedCompanyId);
     executionWorkspaceDefaultProjectId.current = null;
 
@@ -681,6 +688,7 @@ export function NewIssueDialog() {
     if (newIssueDefaults.parentId) {
       const defaultProjectId = newIssueDefaults.projectId ?? "";
       const defaultProject = orderedProjects.find((project) => project.id === defaultProjectId);
+      const hasExplicitProjectWorkspaceId = newIssueDefaults.projectWorkspaceId !== undefined;
       const defaultProjectWorkspaceId = newIssueDefaults.projectWorkspaceId
         ?? defaultProjectWorkspaceIdForProject(defaultProject);
       const defaultExecutionWorkspaceMode = newIssueDefaults.executionWorkspaceId
@@ -697,7 +705,9 @@ export function NewIssueDialog() {
       setAssigneeChrome(false);
       setExecutionWorkspaceMode(defaultExecutionWorkspaceMode);
       setSelectedExecutionWorkspaceId(newIssueDefaults.executionWorkspaceId ?? "");
-      executionWorkspaceDefaultProjectId.current = defaultProjectId || null;
+      executionWorkspaceDefaultProjectId.current = hasExplicitProjectWorkspaceId || defaultProject
+        ? defaultProjectId || null
+        : null;
     } else if (newIssueDefaults.title) {
       setIssueText(newIssueDefaults.title, newIssueDefaults.description ?? "");
       setStatus(newIssueDefaults.status ?? "todo");
@@ -716,7 +726,7 @@ export function NewIssueDialog() {
       setAssigneeChrome(false);
       setExecutionWorkspaceMode(defaultExecutionWorkspaceModeForProject(defaultProject));
       setSelectedExecutionWorkspaceId("");
-      executionWorkspaceDefaultProjectId.current = defaultProjectId || null;
+      executionWorkspaceDefaultProjectId.current = defaultProject ? defaultProjectId || null : null;
     } else if (draft && draft.title.trim()) {
       const restoredProjectId = newIssueDefaults.projectId ?? draft.projectId;
       const restoredProject = orderedProjects.find((project) => project.id === restoredProjectId);
@@ -742,7 +752,9 @@ export function NewIssueDialog() {
           ?? (draft.useIsolatedExecutionWorkspace ? "isolated_workspace" : defaultExecutionWorkspaceModeForProject(restoredProject)),
       );
       setSelectedExecutionWorkspaceId(draft.selectedExecutionWorkspaceId ?? "");
-      executionWorkspaceDefaultProjectId.current = restoredProjectId || null;
+      executionWorkspaceDefaultProjectId.current = draft.projectWorkspaceId || restoredProject
+        ? restoredProjectId || null
+        : null;
     } else {
       const defaultProjectId = newIssueDefaults.projectId ?? "";
       const defaultProject = orderedProjects.find((project) => project.id === defaultProjectId);
@@ -761,9 +773,9 @@ export function NewIssueDialog() {
       setAssigneeChrome(false);
       setExecutionWorkspaceMode(defaultExecutionWorkspaceModeForProject(defaultProject));
       setSelectedExecutionWorkspaceId("");
-      executionWorkspaceDefaultProjectId.current = defaultProjectId || null;
+      executionWorkspaceDefaultProjectId.current = defaultProject ? defaultProjectId || null : null;
     }
-  }, [newIssueOpen, newIssueDefaults, orderedProjects, setIssueText]);
+  }, [newIssueOpen, newIssueDefaults, orderedProjects, selectedCompanyId, setIssueText]);
 
   useEffect(() => {
     if (!supportsAssigneeOverrides) {
@@ -815,6 +827,7 @@ export function NewIssueDialog() {
     setIsFileDragOver(false);
     setCompanyOpen(false);
     executionWorkspaceDefaultProjectId.current = null;
+    initializationKeyRef.current = null;
   }
 
   function handleCompanyChange(companyId: string) {
@@ -1060,7 +1073,12 @@ export function NewIssueDialog() {
   }, [orderedProjects]);
 
   useEffect(() => {
-    if (!newIssueOpen || !projectId || executionWorkspaceDefaultProjectId.current === projectId) {
+    if (
+      !newIssueOpen ||
+      !projectId ||
+      selectedExecutionWorkspaceId ||
+      executionWorkspaceDefaultProjectId.current === projectId
+    ) {
       return;
     }
     const project = orderedProjects.find((entry) => entry.id === projectId);
@@ -1069,7 +1087,7 @@ export function NewIssueDialog() {
     setProjectWorkspaceId(defaultProjectWorkspaceIdForProject(project));
     setExecutionWorkspaceMode(defaultExecutionWorkspaceModeForProject(project));
     setSelectedExecutionWorkspaceId("");
-  }, [newIssueOpen, orderedProjects, projectId]);
+  }, [newIssueOpen, orderedProjects, projectId, selectedExecutionWorkspaceId]);
   const modelOverrideOptions = useMemo<InlineEntityOption[]>(
     () => {
       return [...(assigneeAdapterModels ?? [])]