MultitabController<T> is a Flutter-free ChangeNotifier owning the
tab list, active selection, and reorder/close invariants:
- pinned (non-reorderable) entries form barriers that other tabs
cannot cross
- non-closeable entries silently no-op on remove() so hosts don't
need to gate the call site
- closing the active tab falls right, then left, then to null
- duplicate ids are rejected
MultitabPane<T> is the widget shell: a horizontal tab strip
followed by the active entry's body. Active tab gets the
panelHeader background and a panelActiveBorder top accent;
inactive tabs blend into the tab bar. Close × is hidden until
hover. Add button only renders when onAddRequested is wired.
Hosts route the user's add/close intent through callbacks so the
widget stays domain-free — for the Claude pane, add will spawn a
new tmux session and close will kill one. Drag-to-reorder is
controller-side only for now (the gesture wiring lands with T-24).
19 controller tests + 9 widget tests.
Co-Authored-By: Claude <noreply@anthropic.com>
