Request the windowing system to make window partially transparent, with opacity 0 being fully transparent and 1 fully opaque. (Values of the opacity parameter are clamped to the [0,1] range.) On X11 this has any effect only on X screens with a compositing manager running. See Widget.isComposited. On Windows it should work always.
Note that setting a window’s opacity after the window has been shown causes it to flicker once on Windows.