v0.27.4

TabBar joins the dual-channel pattern with SelectionChanged business events; Button and TextInput move their internal handlers off the user GestureHandler slot. ui! on EventKind accepts a callback-form (on Tap(cb) / on Tap(2, cb)) so handlers can stay one line when there's no inline body.

Added

• TabBarEvent::SelectionChanged { new: u8, old: u8 } + TabBarHandler — fires from tabbar_handler after a Tap moves selected. Same-tab re-tap doesn't emit. Indicator tween starts from the handler itself; tab_pages_system keeps a detect_programmatic_selection_changes pass for callers that mutate TabBar.selected directly.
• ui! { TabBar (...) on SelectionChanged { ... } {} } — handler bodies see new and old auto-destructured from the variant. Qualified on TabBar::SelectionChanged works too.
• Callback-form on EventKind(cb) — the trailing arg in on EventKind(args, cb) may be a callable expression instead of a { ... } body. Combinations: on Tap(cb) (bare callback), on Tap(2, cb) (count=2 callback). Body-form and callback-form coexist on the same widget.
• gallery/examples/tabbar_selection_demo.rs — typed-widget TabBar with on SelectionChanged callback updating a live label.

Changed

• Button and TextInput move to View::with_internal_gesture — button_handler and textinput_gesture_handler no longer install a user-facing GestureHandler component. A user that attaches their own GestureHandler on these widgets no longer overwrites the press / focus-on-tap behaviour. text_input_attach still installs Focusable and KeyHandler on the focus channel.
• TabBar internal handler moves to View::with_internal_gesture — same pattern as Slider / Switch / Checkbox / ProgressBar in v0.27.2 / v0.27.3.

Internal

• xrune dependency: 1.5.2 (1.5.0 yanked previously). DsOn.body is Option<syn::Block> to support callback-form parsing; the macro side handles the optional body.