Alpha / transparency in a gtk_drawing_area

Hello fellow developers,

currently I'm trying to design my first custom widget. The widget is supposed to
graphically represent a rectangular area within a larger area (the widget
boundaries). The user is supposed to be able to drag, resize, and rotate the
rectangle with the mouse, but until the changes are committed I want the
original size/position of the rectangle to remain visible as a semi-transparent shape. Just think of having a semi-transparent rectangle in the top layer of a drawing program such as inkscape while dragging around an opaque one with the
mouse underneath.

Is there any way to do this "natively", or do I have to compute the four colors involved (background, background behind transparent, opaque behind transparent, opaque) and the corresponding polygons myself? I've tried to look it up in the
inkscape code but quickly got lost.


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