O.k. MUIA_Window_HasAlpha fails when the window contains "Dtpic.mui" ("incompatible source and destination bitmaps..." in RAMDebug), using just 1 SimpleButton() as WindowContents does work, and I can doubleclick Ambient-icons through the largly invisible border of my test-app.

Seems I need to write my own alpha-enabled replacement for "Dtpic.mui", something that was planned anyways as I also need scaling