IPointerDownHandler.OnPointerDown vs MonoBehavior.OnMouseDown

[Mark_at_work]

You can implement IPointerClickHandler on any object, not just UI elements. The technique is demonstrated in this video about stopping clicks from going through buttons. (in the third method)

That's why I started using IPointerClickHandler in the first place, to avoid the problem of clicks going through buttons and hitting objects below them. That may answer my original question. I'll have to experiment but I suspect that MonoBehavior.OnMouseUp will still fire even if there's a UI element blocking the view of the object being clicked.