Two fixes to the just-shipped collapse toggles:
- They lived inside the centre StatusbarHost, so they shifted when a pane
collapsed and the bar resized. Move them to the OUTER status-bar row ends
(screen edges) so they stay put. StatusbarHost reverts to its prior layout.
- The toggle was a const widget that read isCollapsed in its own build; Flutter
skips identical const children on rebuild, so the chevron never flipped. Pass
collapsed/visible in from the parent's arrangement listener so it rebuilds and
the chevron tracks the action direction. A 'flips live' test now guards it.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>