While investigating 6365799, I noticed that top-level JMenus (meaning JMenus that
have a JMenuBar as a parent) are rendered differently in Swing's GTK L&F when compared
to native. I only looked into it briefly, but I think GTK themes/engines may return
different x/y thickness and "horizontal-padding" values depending on whether the
menu item is in a menu bar or not. I'm not positive about this, but I do know that
Swing's top-level JMenus take up less space horizontally than their native equivalent,
making them look a bit cramped, which is why I suspected a difference in insets
values here.

Also, GTK themes/engines may choose to render the menu item differently
in a menu bar context (e.g. Clearlooks and ubuntulooks/Human seem to
not round the bottom corners in native apps [see ubuntulooks_style.c, draw_box(),
"menuitem" case], but in Swing's GTK L&F we get the rounded corners in this case
because we don't differentiate the two cases).