Plan step 35: <x-supporting-pane>, following the reference's placement table - below the focus pane below expanded, beside it from 840px at a fixed 360px (412px from large) or the two-thirds split. Below expanded compact="sheet" docks the supporting pane as a bottom sheet whose drag handle opens it; always on the trailing side, so focus order matches the panes on screen, and mirrored in RTL. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9