Pages tall enough to scroll showed a ~15px scrollbar; short pages didn't — so the
centered top bar jumped left/right as you navigated. scrollbar-gutter: stable on html
(SPA app.css + the server-rendered share pages) keeps the layout width constant. No-op
on overlay-scrollbar platforms (mobile), which never shifted.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
