Function step_border_color_transitions 

pub fn step_border_color_transitions(
    commands: Commands<'_, '_>,
    tick: Res<'_, Tick>,
    anim: Res<'_, AnimationsActive>,
    q: Query<'_, '_, (Entity, &BorderColorTransition, &mut Visuals, Option<&Visible>)>,
)

Sample every active BorderColorTransition into Visuals::border.color. A border removed mid-flight retires the tween (nothing left to paint).