[codex] Fix new issue autocomplete pointer selection (#6311)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies.
> - Human operators create and edit issues through modal-heavy board UI
workflows.
> - The new-issue dialog embeds markdown editors that render
autocomplete menus through body-level portals.
> - Radix Dialog treats those portal clicks as outside-dialog pointer
events and prevents their default behavior.
> - That prevention made completion items hard or impossible to select
from inside the new-issue dialog.
> - This pull request marks the markdown editor floating autocomplete
menu as allowed dialog-external UI and extends the dialog
outside-pointer handler to preserve those interactions.
> - The benefit is that users can click/tap autocomplete completions
while keeping the existing modal behavior intact.

## What Changed

- Added a stable `data-paperclip-floating-ui` marker and explicit
pointer event handling to the markdown editor mention/autocomplete
portal.
- Updated the new issue dialog outside-pointer guard so editor
autocomplete portals are handled like Radix popover portals.
- Added regression coverage for markdown editor portal markup and new
issue dialog completion selection behavior.

## Verification

- `pnpm exec vitest run ui/src/components/MarkdownEditor.test.tsx
ui/src/components/NewIssueDialog.test.tsx` passed: 2 files, 38 tests.
- Confirmed the branch is rebased onto current `public-gh/master` before
opening this PR.
- Confirmed the diff does not include `pnpm-lock.yaml` or
`.github/workflows` changes.

## Risks

- Low risk. The change is scoped to allowing pointer events from known
body-level UI portals while keeping other outside-dialog pointer events
under Radix Dialog control.

> 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 with repository tool use and local
command execution. Exact hosted context window is not surfaced in this
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
- [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

Screenshot note: this is an interaction/event-handling fix with no
visible UI change; verification is covered by the focused regression
tests above.

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

ui/src/components/NewIssueDialog.tsx

@@ -1221,12 +1221,12 @@ export function NewIssueDialog() {
           }
           // Radix Dialog's modal DismissableLayer calls preventDefault() on
           // pointerdown events that originate outside the Dialog DOM tree.
-          // Popover portals render at the body level (outside the Dialog), so
-          // touch events on popover content get their default prevented — which
-          // kills scroll gesture recognition on mobile.  Telling Radix "this
-          // event is handled" skips that preventDefault, restoring touch scroll.
+          // Popover and editor autocomplete portals render at the body level
+          // (outside the Dialog), so touch/click events on their content get
+          // their default prevented. Telling Radix "this event is handled" skips
+          // that preventDefault, restoring popover scroll and autocomplete taps.
           const target = event.detail.originalEvent.target as HTMLElement | null;
-          if (target?.closest("[data-radix-popper-content-wrapper]")) {
+          if (target?.closest("[data-radix-popper-content-wrapper], [data-paperclip-floating-ui]")) {
             event.preventDefault();
           }
         }}