I have a couple of mouse buttons mapped (Fluxbox) to execute xvkbd commands, for Enter (\r) and Ctrl+U (\Cu), which comes in super handy.
It works on terminals and most apps, but not on GTK3 apps, it became extra noticeable now that Firefox is switching to it too.
I tested this by doing on a terminal:
sleep 3 && xvkbd -xsendevent -text "\Cu"
And quickly putting focus on a text field for a GTK2 app, and it works. When I do it on a GTK3 app it doesn't.
The rest of mouse bindings work fine, so GTK is recognizing the mouse key presses but ignoring the
xvkbd -xsendevent instruction as proved by the above test.
All I found was this mailing list email from 2012 without replies and not much info: https://mail.gnome.org/archives/gtk-list/2012-May/msg00001.html