Please see my comment to the question. Is that what you need?
If so, one of the way to do it is to assign some instance of
of non-rectangular shape to the property
This method is the cleanest one: not only a window looks non-rectangular visually (like it would look with transparency), but is also transparent to mouse events, for example: you click mouse in the corner; and the window of the application beneath is activated — this is something you really need.
It's also good to use
to avoid showing non-client areas of the window.
Another way is related to
] (read: "Non-Rectangular Window Style"),