https://bugs.freedesktop.org/show_bug.cgi?id=106441
--- Comment #6 from Richard B. Kreckel kreckel@ginac.de --- (In reply to Richard B. Kreckel from comment #5)
It must be Xorg.
I've upgradee to Xorg 1.20 (without upgrading anything else) from Debian/experimental and this problem has disappeared for good.
...which is not to say that things work, now. In a way it's worse than before but this seems to be another problem: https://gitlab.gnome.org/GNOME/totem/issues/241. (I'm posting this here for the record.)