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
