The summary says it all. It's most noticeable in browsers such as Firefox or Chrome. This was not a problem in Gnome / KDE. For me, it's a show stopper for XFCE. FWIW, I'm running Debian 8. Sirius ~ uname -a Linux Sirius 3.16.0-7-amd64 #1 SMP Debian 3.16.59-1 (2018-10-03) x86_64 GNU/Linux Note: Sirius is the computer's network hostname.
Please ignore this "bug". It seems that my mouse's middle button chose the moment I switched from Gnome to XFCE to break. I switched back to Gnome, and confirmed it was the mouse. Sorry for the inconvenience!
Closing this report then.