Many thanks for the help.
You're running unstable, these things happen...
Yes, it's why I run unstable - learning opportunities and sometimes able to feed back bug info.
Why on earth would you upgrade your system from synaptic
Seriously, why not? (This is not argumentative, I would like to know the downsides of using Synaptic.)
Now to the meat of things. It seems that the xorg versions for testing and unstable are the same. What worked was removing xorg AND xserver-xorg-core then disabling the unstable repo (I use apt-proxy on another machine) and installing xorg AND xserver-xorg-core.
Install xorg only did not bring in the other packages. Xserver-xorg-core brought in 37 others. xserver-common and xserver-xorg-devs have remained at the unstable versions. I have not tried tinkering with them yet.
The system is now back to normal. For posterity, some of the symptoms I had were:
When using kwrite or kedit: messed up display.
In Dolphin: no little arrows next to openable directories
Checkboxes do not show checks
Radio buttons were invisible.
Tabs in firefox get overwritten
KDE crashes when you open a pdf file with KPDF or Acrobat. Others not tried.
In Calibre, spinning wheel (=working) does not display.
I don't know if this is related: kdesudo stopped working at the same time as the xorg mess happened and still does not work. I may end up asking a new question about that.