Jules, would it be possible to add:
LookAndFeel::drawTabAreaBehindAllButtons(TabbedButtonBar&, Graphics&) {}
and have it be called before any other painter LAF methods.
For custom tab bar LAFs with a solid background, as far as I can tell at the moment, one needs to frap around drawing the background elsewhere (either in tab bar sub-class, or the container). I'd rather not have that kind of style logic hanging around in my code if I can help it.