Re 'severity' :https://www.debian.org/Bugs/Developer#severities states: "Grave: makes the package in question unusable or mostly so" - er.. the package WAS unusable.
but point taken anyway :). After messing around again yesterday; reinstalling, clearing out ~/.mysql, etc. It was still in the same state when I shut down for the night. BUT.. this morning on a random whim, after booting, I used XFCE (rather than Gnome3) and tried mysql-workbench again and it WORKED in XFCE. Now it seems to be working in Gnome again. I can only guess some kind of corruption got 'cleared' but I have no idea what has happened. For the record the issue occurred on the up to date testing - version 6.1.4+dfsg-2 (which is the same version as in unstable). Sorry to have troubled, but as even I cannot reproduce now I guess you should close.
