- GDM background persists as Gnome desktop background and is not changeable.
- Terminal opens black and blank- unusable.
- GUIs fallback to Windows 95 like blockiness
- Screen does not shut down when computer is idle.
Another symptom is that it's not possible to change the GTK+ theme for example in Gnome-Tweak.
I eventually came across the answer on DebianFAQs:
There's a bug in touchpad setttings that means ticking Disable touchpad while typing will break GTK, and the Gnome GUI.