fix(ui): avoid mixed theme preview frames

[benvinegar]

Summary

• keep theme selector open/selected/preview state in one atomic state object
• derive the rendered preview theme without committing it until Enter
• avoid transient mixed-color frames when previewing, accepting, or cancelling theme selection

Verification

• bun test src/ui/AppHost.interactions.test.tsx --test-name-pattern 'theme'\n- bun run typecheck\n\nThis PR description was generated by Pi using OpenAI GPT-5

[greptile-apps]

Greptile Summary

This PR fixes transient mixed-color frames in the theme selector by consolidating three separate state variables (themeSelectorOpen, themeSelectorIndex, and themeSelectorOriginRef) into a single atomic ThemeSelectorState object and introducing a previewThemeId overlay on top of the committed themeId.

• Atomic state consolidation: Replaces three independent state pieces with one ThemeSelectorState object (open, selectedIndex, previewThemeId), ensuring open/index/preview transitions always land in the same render frame.
• Non-mutating preview: Theme previews now write to previewThemeId instead of directly mutating themeId; effectiveThemeId = previewThemeId ?? themeId drives rendering, so cancelling is a single null-assignment with no intermediate frame showing the wrong theme.
• Simplified cancel path: The themeSelectorOriginRef tracking the origin theme is removed — Escape simply sets previewThemeId: null in one update, atomically reverting all previewed colors.

Confidence Score: 5/5

Safe to merge — the change is narrowly scoped to theme-selector state management and is well covered by the existing interaction test suite.

The refactor correctly uses functional updaters for all concurrent-safe paths (move, pick, close), batches selectTheme with setThemeSelectorState in acceptThemeSelector so no intermediate frame can show a mismatched theme, and removes the themeSelectorOriginRef without leaving any dangling restore path. The escape/revert, accept, and reopen-on-committed-theme flows are all exercised by the existing tests in AppHost.interactions.test.tsx.

No files require special attention.

Important Files Changed

Filename	Overview
src/ui/App.tsx	Core refactor of theme selector state: three separate pieces (open bool, index, origin ref) merged into one atomic ThemeSelectorState object with a previewThemeId overlay; logic is consistent across open/move/pick/accept/close paths with correct functional updater usage.
.changeset/theme-preview-flash.md	Adds a patch-level changeset entry describing the fix for mixed-color frame flashes during theme preview/acceptance.

_{Reviews (1): Last reviewed commit: "fix(ui): avoid mixed theme preview frame..." | Re-trigger Greptile}