I believe this is the intended behaviour, MouseDown seems to ignore pixels with alpha 0 (fully transparent). It makes custom clickable shapes very easy to do. So if you never call paint event, there are no clickable pixels. However I do see a use case where you want a clickable canvas that can be clicked on even transparent pixels. So maybe an option to set MouseDown behaviour would be useful. Or just make it always accept mouse down and have the developer handle the clickable regions.