Depending on the effect you're going for, you're going to want different OnButtonUp and OnClick actions. The behavior I settled on was:

button pressed inside - OnDown fires
button pressed inside and released anywhere - OnUp fires
button pressed inside and released inside - OnUp and OnClick fire

I don't remember what exactly I considered, but I think you need events like this to easily simulate OS-native buttons. (OnUp is for visual effects/mouse tracking, OnClick is for execution)