Tryst::Tween

A small animation helper on top of App#every - the "duration, easing, cancel-on-destroy" piece OwnerDrawnWidget's own doc comment calls for. App#every itself is just a flat, unbounded repeating interval with no concept of a start/end or a progress value; Tween adds exactly that on top, ticking at a fixed ~60fps until duration_ms has elapsed, calling block with the EASED 0.0-1.0 progress each tick (1.0 on the final tick, guaranteed even if the last interval overshoots).

Cancel a running Tween with #cancel - OwnerDrawnWidget wires this to App#on_widget_destroyed so an in-flight animation on a destroyed widget can't fire into freed state (see OwnerDrawnWidget's own doc comment on why that hook, not just its own #destroy, is what a timer needs to be cancelled from).

Tween.new(app, duration_ms: 200, easing: :ease_out_quad) do |progress|
  handle_width = (start_width + (end_width - start_width) * progress).round.to_i
end