Disable touchpad while typing breaks GTK

I've noticed some strange behaviour on Debian Wheezy recently, as I described on the Debian forum:

• 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.