Sometime recently on master, GTK CSD windows regressed in resizability. The cursor doesn't change to a resize arrow near the edges of their windows, nor right on the single pixel border either. Tested with Gedit and Peek.
Reproduces for me on X11, but not Wayland.
Of course, after logging out and logging into Wayland to test, and then going back to X11, now it's working again. :/