Invoked to represent unselection.
This event should be invoked when the input state is observed to represent the user's intent to stop selecting, the conditions for which vary by selection mechanism; for example, IndexPinchSelector invokes this event when the user's index finger and thumb are observed to go from "pinching" to "not
pinching." Contractually, there must be a 1:1 relationship between
WhenSelected and WhenUnselected invocations, with one of the latter always following one of the former.