Apparently XFCE 4.16 has adopted Gnome's horrible idea of custom window decorations / client side decorations (CSD) making it look completely out of place for those of us who designed and use our own custom window decoration theme.
This is a big step backwards, towards the ugliness of Gnome, where every app looks differently and behaves differently from the rest of the system, which is something many of us have been fleeing from.
Is there a switch or a setting to disable this behaviour?
Currently only XFCE dialogs and Gnome native applications (which I try to use as little as possible) have this monstrosity. But I fear it's a plague that's spreading.
(I blame iTunes, but that's just a historical note.)
You cannot as such disable CSD, Client Side Decorations, because these are provided by the "client", i.e., the software itself. You discovered the package
gtk3-nocsd, which enables legacy window decorations on CSD applications, but the result is not always optimal, although still better depending on your personal preferences.Alternatively, where possible, you may choose to select and use alternative software that does not use CSD. The Mate desktop has alternatives for the likes of Gedit (Pluma), Evince (Atril), Eog (Eom), which are forks that use legacy window decorations and a traditional menu bar. The XApps project of the folks over at Linux Mint also is aimed to produce applications for traditional GTK desktop environments, using modern toolkits but traditional interfaces, designed to be desktop- and distro-agnostic (e.g. Xed, xreader, xviewer).