Keep the GitHub issue composer within reach

[Harry19081]

Problem

GitHub issue comments sit at the end of long threads, mix issue-level status actions into the text-input footer, and allow users to select the already-current status. Duplicate-search loading also uses a full loading bar inside a compact dropdown.

Solution

Pin the issue composer to the bottom of the thread with measured content padding, move issue-level status actions above the comment input, use compact upward-opening formatting controls, disable the current status option, and use an inline spinner for duplicate-search loading. Extend property options with a native disabled state and keep other work-item editors on the same compact toolbar configuration.

Potential risks

The floating footer reserves at least 240 px and then tracks its measured height; unusually tall composers may reduce visible thread space until they collapse. Closed-issue coloring consumes the purple theme variables introduced by PR #716. This PR is stacked on PR #719 for the rich-text toolbar API and should merge after both dependencies. Manual narrow-panel, keyboard, and theme evidence is pending, so this PR remains a draft.

Performance guard

Verdict: pass. A footer owns one ResizeObserver and one existing resize listener only while mounted; enabled: false prevents ownership when no footer exists, and cleanup is asserted on unmount. Hidden, idle, and repeated-open behavior adds no timer, poll, subscription, cache, or retained observer. Multi-instance behavior is per mounted thread and bounded to one observer per visible composer.

Audit

The configured frontend-ui-audit skill file was unavailable at both documented locations. A direct pass confirmed reuse of Button, Dropdown, PropertyField, RichMarkdownEditor, and existing semantic tokens. Current status options are native disabled buttons with aria-disabled, and the floating composer remains a labeled section.

Verification

• Vitest for PropertyDropdownField, GitHubIssueComposer, GitHubIssueThreadSurface, and WorkItemThreadLayout — passed (16 tests across 4 files).
• ESLint on all 17 changed TypeScript/TSX implementation and test files — passed.
• pnpm typecheck — passed after correcting the strict React test fixture.
• git diff --cached --check — passed before commit.
• Manual desktop verification was not run because local UI control was not authorized for this task.