fix(cli): resolve autoThemeSwitching when background hasn't changed but theme mismatches

[sehoon38]

Summary

Fixes an issue where autoThemeSwitching would not work if the terminal background color hadn't changed since the application started, but the active theme was mismatched (e.g. a Light theme on a Dark background).
The bug was caused by an early return inside handleTerminalBackground that bypassed the theme-switching logic when previousColor === hexColor.

Details

Moved the logic for evaluating the theme (getLuminance, shouldSwitchTheme) to happen before the early return check, so the theme matching logic still runs and correctly applies the theme even when the background hasn't changed.
Included a new unit test in useTerminalTheme.test.tsx to explicitly verify this edge-case behavior.

Related Issues

Fixes #20705

How to Validate

1. Open Gemini CLI in a terminal with a dark background but with your ui.theme set to default-light (and autoThemeSwitching enabled).
2. Start the CLI. Wait for the terminalBackgroundPollingInterval (default 60s).
3. Observe that the theme now correctly switches to the default dark theme, instead of remaining permanently stuck on light.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a bug in the autoThemeSwitching feature of the Gemini CLI. Previously, the system would fail to switch themes if the terminal's background color hadn't changed since startup, even if the current theme was inappropriate for the background (e.g., a light theme on a dark background). The changes ensure that theme matching logic is always executed, allowing the CLI to correctly apply the appropriate theme based on the terminal's luminance, thereby improving user experience and consistency.

Highlights

• autoThemeSwitching fix: Resolved an issue where autoThemeSwitching failed to apply the correct theme when the terminal background color remained unchanged but the active theme was mismatched.
• Logic reordering: Reordered the theme evaluation logic (getLuminance, shouldSwitchTheme) to execute before the early return check in handleTerminalBackground, ensuring theme matching runs regardless of background color changes.
• Test coverage: Introduced a new unit test to specifically validate the corrected behavior for the autoThemeSwitching edge case.

Changelog

• packages/cli/src/ui/hooks/useTerminalTheme.test.tsx
  • Added a new test case to verify theme switching when the terminal background is identical but the current theme is mismatched.
• packages/cli/src/ui/hooks/useTerminalTheme.ts
  • Moved the getLuminance and shouldSwitchTheme calls to occur earlier in the handleTerminalBackground function.
  • Modified the early return condition to allow theme selection to proceed if a new theme is determined, even if the terminal background color has not changed.
  • Relocated the config.setTerminalBackground and themeManager.setTerminalBackground calls to execute only when the background color actually changes.

Activity

• Added a new unit test to cover the identified edge case.
• Validated the fix on MacOS using npm run.
• Linked to related issue Fix autoThemeSwitching when terminal background color hasn't changed but theme is mismatched #20705.

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[gemini-code-assist]

Code Review

This pull request correctly resolves an issue where automatic theme switching would fail if the terminal background color hadn't changed, but the theme was mismatched. The fix is well-implemented and is accompanied by a new unit test that specifically validates this edge case. The suggested refactoring for improved conciseness and maintainability has been retained as it aligns with general best practices.

[github-actions]

Size Change: +95 B (0%)

Total Size: 25.8 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	25.3 MB	+95 B (0%)
./bundle/node_modules/@google/gemini-cli-devtools/dist/client/main.js	221 kB	0 B
./bundle/node_modules/@google/gemini-cli-devtools/dist/src/_client-assets.js	227 kB	0 B
./bundle/node_modules/@google/gemini-cli-devtools/dist/src/index.js	11.5 kB	0 B
./bundle/node_modules/@google/gemini-cli-devtools/dist/src/types.js	132 B	0 B
./bundle/sandbox-macos-permissive-open.sb	890 B	0 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB	0 B
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB	0 B
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB	0 B
./bundle/sandbox-macos-strict-open.sb	4.82 kB	0 B
./bundle/sandbox-macos-strict-proxied.sb	5.02 kB	0 B

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