Created attachment 772 [details]
screenshot of issue
Only seen on Safari on Mac OS. Seen mostly when moving windows around. Gets worse when toggling compositing on or off.
This seems to be fixed in new versions of Safari. Can't reproduce on macOS 10.13.1 with Safari 11. Tested with an ubuntu 16.04 server with MATE with and without compositing and with eudemo with GNOME 2.