If, for example, you have a 100x100 Rectangle, you can't set its topLeftRadius to 75. The corner radii are restricted to half the width/height.

Obviously there is no inherent reason to not allow a 75px round on one corner, it's just an artifact of the current implementation, and it's quite frustrating.

The drawing code should be improved to allow any arbitrary corner radii as long as they don't conflict. I would recommend just doing whatever HTML does, so it works as people expect.