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Created attachment 6932 [details]
How Control.Region works on Windows
Control.Region adjusts where the control draws itself, but doesn't change the shape of the control itself. e.g.:
Label lb1 = new Label();
lb1.Region = new System.Drawing.Region(new System.Drawing.Rectangle(0, 0, 15, 7));
On Windows / .Net it only displays the part of the label inside the region. The parts of the label clipped by the region are not drawn at all, do not obscure other controls, nor affect said other controls' interaction with the mouse pointer. See the attached example Control.Region Windows.
On Linux it only displays the label's background colour and text inside the region, but the entire border and the parent's background colour are still drawn, and those parts obscure any other controls that it is in front of. See the attached example Control.Region Linux.
I think one fix would be to use the "X Nonrectangular Window Shape Extension Library" (http://www.x.org/releases/current/doc/libXext/shapelib.html) in XplatUIX11.cs.
Created attachment 6933 [details]
How it works on Linux