You nailed it man.
That bug report led me to the RHEL bug: https://bugzilla.redhat.com/show_bug.cgi?id=1305369
It in turn led me to the following workaround:
- Open SMPlayer, go to Options -> Preferences
- Go to "Advanced", then to the "Options for MPlayer/mpv" tab
- add
-vo gl
to the (empty) option list
Problem fixed in SMPlayer.
I still can't exactly understand the correct package to file the bug against, but at least it will add some meat to the bug report.
Thanks a bunch mate.
I didn't do anything more than the classic "Driver & Firmware" step of Welcome -> Getting Started
Edit: Reported bug here: Bug #1619556 “Video Stacking/Scaling issue on DELL OptiPlex 7440...” : Bugs : xserver-xorg-video-intel package : Ubuntu
Marking this thread as solved since the goal was to find how to report the bug.