Plan step 19, containment.md C-11. The full-screen layout was another horizontal row with a 28px corner and a mask, where M3's "shows one edge-to-edge large item at a time and scrolls vertically". It is now a vertical scroll-snap column: items fill the row with no corner and no mask, 16dp apart, no end padding, the previous/next buttons point up and down and the arrow keys are Up and Down. The row is capped at the 840px medium window, which with the portrait rule in the header comment is as far as CSS can hold M3's "compact and medium, portrait only". The ported FullScreenCarouselStrategy keylines go with it: nothing measures them any more. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9