What I have in mind is a small canvas (with transparent background and now window decoration) that pops up under the cursor when a mouse button is pressed, and upon which canvas items like circles, polygons, etc. are drawn to act as control elements. I know I can do everything I need to do to make this work except put up a mainwindow whose background has a stipple of "transparent", and containing a canvas which also has a transparent background.

That's the key element I can't seem to figure out how to do (and suspect can't be done without me doing something drastic like writing my own version of a canvas and/or mainwindow).

If I can't do it, I'll have to settle for the less aesthetically pleasing (and content occluding) grey background.