Disasterous :( For me, it has begun from evince and deadbeef, now firefox is on its way to follow. The problem is that I am unable to find any compact gtk-3 theme that looks like xfce-b5 theme for gtk-2. Xfce theming engine seems to be broken under gtk>=3.8. The only possibility is to recreate theme from scratch and pray that nothing would be broken when minor bumps again. Hell, I am unable to find any theme compact…
What's the problem with deadbeef? It supports both GTK+2 and GTK+3 interfaces - GTK+2 is even the default and considered more stable since most of deadbeefs developers prefer that one. Edit: Regarding Evince, there's Atril. It's the GTK+2 evince fork from the mate developers.
Regarding deadbeef, yeah, it was mentioned mostly on emotional basis. I had some troubles building gtk2 plugin on my home machine, good chances that it was problem with inconsistent building environment, yet no problems with building simple gtk2 apps or deadbeef gtk3 plugin. I haven't dug deeply though. At work, I just use slightly older deadbeef version with gtk2 plugin :)