TapHandler needs a margin property (be able to press outside the parent Item's bounds)

This is convenient for Fitts's Law compliance: if you have placed a button on some sort of panel near the edge of the screen, you should be able to press the button even when the mouse cursor is exactly at the edge of the screen.

So far our answer has been that you can make another (invisible) Item with appropriate bounds, just to host the TapHandler. But that takes more memory and is less convenient.

The danger with adding a single "margin" value is that next, people may start asking for it to have separate margins on all 4 sides. That might be a waste of memory, given that most handlers don't need them; but adding a single qreal property is not as bad.