I’ve been playing with c# and Windows Forms programming lately, and found a couple articles about making custom non-square windows easily by hiding the frame and making the part of the square you want to get rid of transparent. Super easy to do. Of course, using this technique you still really have a square window, which your users will discover when they are trying to click on a window behind yours to switch focus to another app, and they are really clicking on the invisible part of your window. Still, this would be great for splash screens or windows that don’t really have that much area that you want to make transparent. To really do it right you should research setting the region of your form to a GraphicsPath, but this is much easier.
Anyways, here’s a link to one of the tutorials.