A caveat: GTK3 brought a lot of changes in GTK/GDK internals including how window sizing was handled, both based on widget size requirements and based on user interactions for resizing. This was intended in part to provide better support for WM paradigms like Windows and Wayland where clients have more control of their own size, and resulted in UI additions like resize grips by default in the corner of windows. It also cleaned up the code to be less semantically tied to X11's windowing rules. In many cases it's more efficient to use a single toplevel window and do logic application-side than make the X server work harder (due to context switches and changes in rendering codepaths).

The downside to these changes is that several GTK3 features were horribly broken outside of GNOME's default window manager. I've seen at least five or six window managers that would freeze applications with the mouse grabbed as of GTK3.2: click on the wrong part of a menubar or on the resize corner and you had to resort to the keyboard to kill the offending application before it or your mouse would work. Version 3.4 or thereabouts has fixed these bugs for the most part; even window managers that don't support much in the way of ClientMessage-based window manager protocols should work fine now. But for some time GTK3 did interact very poorly with most window managers.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=630972

That this was so hairy and bug-prone is, in my opinion, a symptom of interacting with the ugly beast that is X11. If you read the code of the affected window managers and GTK, you would probably agree.