Fixed ~24px caret-line cells bookend the status bar (left=sidebar, right=context); chevron flips per arrangement.isCollapsed (inward=collapse, outward=expand) and fires the existing sidebar.collapse / context.collapse commands — the mouse affordance for the already keyboard/CLI-addressable action (D-6). Visual only; no new collapse logic. Status items shift 28px inward for the reserved cells. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>