See attachment. Video collected during https://www.xpra.org/trac/ticket/2029?#comment:14
This can happen if for some reason we don't get any data through.
The whole thing will need to be re-written for python3.
Re-write for GTK3: #2195
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2045