Comment 18 for bug 1806292

Its a native GtkMenuBar inside a toplevel GtkWindow. So presumably the min size is that of its contents. GtkToolBar comes with a "show-arrow" property which allows the toolbar to be shrunk past the size of its contents and show the entries in an overflow menu. I see no such properties with a GtkMenuBar, so I think that's just the way it is with a native gtk menubar so someone probably needs to ask upstream gtk if there's a mechanism to achieve this, if it even is something that should be desirable. Sticking it into a scrolledwindow would just leave the excess elements unusable.