fix(desktop): isolate hidden pane composer metrics

[qdivan]

Summary

• prevent kept-alive hidden Desktop chat panes from publishing shared composer height metrics
• make document-root composer metric cleanup owner-aware so a background pane cannot clear the foreground pane's values
• add focused component regressions for hidden-pane measurement and unmount behavior

Addresses #71264.

Validation

• RED on origin/main: NODE_ENV=test npm run test:ui -- src/app/chat/composer/hooks/use-composer-metrics.test.tsx (2 expected failures: hidden measurement overwrote 88px with 32px; hidden unmount cleared the foreground value)
• GREEN: NODE_ENV=test npm run test:ui -- src/app/chat/composer/hooks/use-composer-metrics.test.tsx (3 passed)
• NODE_ENV=test npm run test:ui -- src/components/pane-shell/pane-visibility.test.ts src/app/chat/composer/focus.test.ts src/app/chat/session-drag.test.ts src/app/chat/composer/hooks/use-composer-metrics.test.tsx (11 passed)
• npx eslint src/app/chat/composer/hooks/use-composer-metrics.ts src/app/chat/composer/hooks/use-composer-metrics.test.tsx
• npm run typecheck
• git diff --check

Duplicate Check

Checked live on 2026-07-26 against origin/main at 07e97d2f5dc3d2092cfe693ef07b2527a36cd2d8. Issue #71264 remains open with no linked PR or comments. Searches by issue number, useComposerMetrics, composer-measured-height, and hidden-pane metrics found no equivalent active or merged fix. PR #39564 addresses separate scrolling/stacked-layout behavior and does not modify the shared metric hook or hidden-pane ownership.

Browser / Playwright

Not run: this is a Desktop/Electron issue reported on Windows 11, while the validation environment is Linux. The component regression uses the production data-pane-hidden marker and directly exercises the document-root CSS variables that control thread bottom clearance. A Windows 11 Desktop smoke test remains useful for final pixel-level confirmation.

[qdivan]

Closing as superseded by #71840, which has now merged a broader per-chat-surface fix for the same global measured-height race (including composer cleanup ownership). Thank you!