[Bug] Claude added overflow-x-auto to a fixed-height tab strip, drawing a scrollbar beside the tabs

Bug Description
Asked to stop a page's tab strip from widening the page on phones, Claude put
overflow-x-auto on a fixed-height shadcn TabsList (h-9). Setting overflow-x
also turns overflow-y from visible into auto, so on macOS with "Show scroll bars: Always" a
vertical scrollbar appeared beside the tabs on desktop. Claude reported the change as
verified: its headless Playwright check hides scrollbars until you scroll, so the
screenshot never showed it.

Steps to reproduce

1. Add overflow-x-auto to a shadcn TabsList (fixed h-9).
2. View it in Chrome on macOS with System Settings → Appearance → Show scroll bars: Always.
3. A scrollbar thumb sits beside the tab buttons.

Expected
A strip of buttons or tabs wraps (flex-wrap + h-auto) instead of scrolling. UI checks
should measure scrollHeight > clientHeight / scrollWidth > clientWidth rather than
rely on screenshots taken with hidden scrollbars.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.283
• Model: claude-opus-5-5
• Feedback ID: dcf4d394-a71b-4758-96d8-14b928d5bec1 (earlier report: 0270d792-ecca-456b-98ce-426e58ff0522)

Errors
None. The code was valid CSS; the problem was visual.