Modern UI - Double border for the integrated browser

[lppedd]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: Version: 1.131.0-insider (user setup) Commit: a816e1a
• OS Version: Windows 11

[vs-code-engineering]

Sharing media and context from merged PR #335463 for verification:

Summary

• let the Modern UI editor card own the integrated Browser's resting boundary
• retain the Browser-specific border reservation for classic UI, modal editors, agent sharing, and device emulation
• cover fractional-zoom rounding at the editor-card edge and add focused regression coverage

Validation

• npm run compile
• ./scripts/test.sh --runGlob '**/webContentsViewRendererFeature.test.js'
• ESLint on the changed production and test files
• targeted stylelint/design-token hygiene on Browser and Modern UI boundary styles (no errors)
• real UI checks with Modern UI on/off; dark, light, and high-contrast themes; focused and inactive split Browser editors; zoom levels -1, 0, and +1; window resize; rounded corners; and device emulation

Before / After

Both captures use the Browser welcome page, Dark Modern, Modern UI enabled, a 1440×900 window, zoom level 0, hidden sidebars/panel, and the focused URL field.

Context	Before	After
Full window
Border detail

Source PR: #335463

[lppedd]

Lee Murray (@mrleemurray) seen the revert, could we reopen the issue? Mostly for my own tracking.