A fix for this has been done in 8.0_1.34
(Currently in PSO experimental, but should appear in the normal repos in a few days).
The fix detects Xubuntu (well actually XFCE), and sets some GDK settings that fix this issue.
Other Linux Systems
If anyone is seeing this error on other Linux systems, although I still recommend that you use WASTA, one can try the one of the following options (in version 8.0_1.34 and newer):
GTK_USE_POPUP_WINDOW=1 paratext8
GTK_USE_TOPLEVEL_WINDOW=1 paratext8
(The latter is default on most systems)
And while I’m publicising internal environment variables:
FORCE_GRAB_KEY=1 paratext8
switching the keyboarding/IM implementation to a keyboard grabbing approach. (this is not the default on most systems)