Function drain_pending_events 

pub fn drain_pending_events(
    events: &Arc<Mutex<EventState>>,
    canvas_size: (u32, u32),
    focused: bool,
    mouse_down: bool,
    mouse_pos: (f32, f32),
) -> Vec<(u32, String, Vec<u8>)>

Drain built-in event sources + the pending custom-event queue and return (callback_id, type, data) tuples to dispatch via on_event.

state is mutably accessed once. The runtime then calls on_event for each returned tuple, after writing the type/data into EventState::current via set_current_event.