I've tried to sort this myself with various internet searches, video card settings, and setiings (including a number of video output drivers) within SMPlayer to no avail.
Linux Mint 19 with AMD HD7790. SMPlayer 18.6.0 rev 9075
When watching a video, whenever I right click on the screen or move down to load floating control (not animated helped a bit), or adjust anything on floating control, the entire screen flickers black before returning to otherswise smooth play. SMPlayer is the only player affected.
Any suggestions?