I've got a component "x" with a Window containing a Tabbed Pane.
I made the background of the tabbed pane transparent in order to hide stuff inside that would be higher than the tabbed pane (actually the transparency only makes sure the bottom border of the tabbed pane is drawn over the stuff inside, so I add a white borderless panel just below the tabbed pane as well).
Unfortunately, in a new screen I can only put the whole component "x" in front or back of other stuff. So I need the Window widget to have a transparent background as well (such that I can put the component "x" in front of all the other stuff which should still be visible through the tabbed pane and window).
I hope you get the idea. Another helpful feature would be to allow container widgets limit the visibility of stuff inside. Maybe by really making this widgets function as containers (i.e. you can double click them to put other widgets inside). That would make my workaround described above redundant.