Background layout preferences aren't specific to light or dark modes

[rzhao271]

Testing #333232

I right-aligned the background image for dark theme. I then tried choosing the same image and left-aligning it for light theme, but the dark theme image also became left-aligned.

[TylerLeonhardt]

🤖 AI verification on 9a754577e07fd950d724afedf9e9f5e0530a6efa (origin/main): reproduced.

Using an isolated Agents window, I set the same image to Right in Dark Modern, switched to Light Modern, selected that image from recently used, and changed its layout to Left. After switching back to Dark Modern, the dark background was also left-aligned rather than retaining Right.

The persisted settings confirm the behavior: the dark and light image paths remain separate, but both use the single chat.agentSessions.backgroundImageLayout value, which changed from right to left in the light theme. This verifies the report, though the shared layout appears consistent with the currently documented single-setting model in the parent TPI; whether layouts should become theme-specific is therefore a product decision.

[vs-code-engineering]

Sharing media and context from merged PR #333873 for verification:

Summary

• store dark- and light-theme Agents chat background image layouts independently
• reload the active layout when the color theme changes
• keep image and layout preferences machine-local and excluded from Settings Sync
• update accessibility help to clarify that layout changes apply to the current color theme

Testing

• npm run transpile-client
• .\scripts\test.bat --run src\vs\sessions\services\chatBackground\test\browser\chatBackgroundService.test.ts (11 passing)
• npm run eslint -- src\vs\sessions\services\chatBackground\browser\chatBackgroundService.ts src\vs\sessions\services\chatBackground\test\browser\chatBackgroundService.test.ts src\vs\sessions\contrib\chat\browser\chat.contribution.ts

Visual verification

Verified in an isolated native Agents window:

1. Dark Modern image set to Right.
2. Light Modern image set to Left.
3. Switching back to Dark Modern restored Right.

Light Modern — Left

Dark Modern — Right Restored

Source PR: #333873