Effectively do the same as in the past when we do not have a headers group.
Trigger for this is that the line does not work in such cases but we also
do not need to draw the background in any special way then.
It can happen that an application uses a QMainWindow in its widget hierachy for
its features but does not correspond to an actual window. Only alter those
toolbars for the unified look whose QMainWindows are actual windows and as such
have a window decoration.
BUG:427410
David Redondo
Carson Black