We already did this with sidebars in settings windows (T11279), to great effect and acclaim. Now they look fantastic:
Kirigami also uses this style for desktop apps created with it, and it looks great. For visual appeal and consistency's sake, it's time to bring this style into more of our QWidgets apps, by putting similar single-pixel lines in the following places:
- Underneath toolbars, to separate them from content views below them
- Separating QDockWidgets from main views beside them
- Separating other sidebars more generally from content views beside them
- Separating status bars from content views above them
Then we should remove the "framed" appearance from content views, since it's no longer necessary once they're framed by the these single-pixel separator lines and the window edges.
The idea is to reach something like the following for Dolphin: