Collapsed side panels render as a 12px vertical spine with rotated label, hover highlight, and badge dot (D-051). Focus mode snapshots and restores the full layout arrangement (D-052). Fourteen new commands in the default-layout extension: collapse toggles (Ctrl+Shift+1/3), panel focus (Ctrl+1/2/3), focus mode (Ctrl+.), sidebar section switching (Alt+1-5), Escape dismiss (D-054). Right panel gets a bottom icon rail matching the sidebar pattern (D-047). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>