Bug#923334: I suspect it's not an xorg problem
I went into IceWm after reporting this this morning and xorg is acting normally under that windown manager. I now suspect it's a Mate problem. Sorry for the noise.
Bug#923260: xserver-xorg: another upstream bug report
Package: xserver-xorg Version: 1:7.7+19 Followup-For: Bug #923260 A bug report in upstream (for Fedora 29) with the very same description: https://bugs.freedesktop.org/show_bug.cgi?id=109174
