Comment # 6 on bug 106441 from Richard B. Kreckel

(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.)