Use of an $OnAdd/$OnRemove action
When a note is created on top of an adornment, the adornment's $OnAdd action is run; prior to this, the adornment action ran only when the note was moved onto the adornment. The adornment's $OnAdd is also run on other adornments placed in or overlapping the adornment if the former is in front; if behind no $OnAdd fires.
$OnAdd also affects adornments created or pasted into containers and the adornment's $OnRemove action is performed if a note atop it is removed or deleted.