It's subtle, but as you can see in the screenshot:
there are no min, max or close buttons on the windows. The edges of windows can be resized, but it doesn't help the situation. Clicking on a window behind another window doesn't bring the background window to the foreground. There's no way to "focus" on a particular window.
The alt-tab shortcut to cycle between windows is non-responsive.
Also, some menus seem to be missing some options. In the pan newsreader, when composing a message, there's no option to paste text anymore. It almost seems as though a piece of the window manager is missing.
I'm running on old hardware, and only have xfce installed, can't support much more. Everything was fine last night until logging out, but this is now persisting as a problem.
I will look at X logs, but right now have now way of copy/pasting them to this question, so I'll edit that in later as I figure out how to work around not being able to copy/paste between applications.