If a menu item in a submenu is part of a section, it should have
rounded bottom corners if both the item and the section are the
last child of its respective parent.
To express that, add a new .popup-menu-section class and use that
to undo/redo the rounding for items inside a section.
It would be possible to do without a new class with a selector like
> StBoxLayout > .popup-menu-item:last-child:hover,
:last-child > .popup-menu-item:last-child:hover
but that's hardly better with its heavy reliance on implementation
details.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4940
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2126>
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