Ubuntu 18.04. Well familiar with SMplayer over several years. when I try to use it now, the video opens in a second window with no controls. I have to click on the SMplayer window to operate controls and then the video hides and I have to click to bring it back. The second window is not resizeable. I saw a similar question with over 300 views, but the answer given as a possible one does not work. I have searched the web and SMplayer forums, but can find nothing. Last ditch effort as i may have to give up as SMplayer is useless with this issue.
SnapOverflow
Launch SMPlayer and go to Option > Preferences > General > Video (tab). Then select "x11" for Output driver and click on the "Apply" button.
If it doesn't work try selecting another option for Output driver.
Problem resolved.
I was able to track the opening 2nd window to mpv player. So in the SMPlayer settings, I changed the player to mplayer. However, an error message asked for the path to mplayer which led to the discovery that mplayer was not installed. So neither Ubuntu nor restricted extras installs mplayer any more. A separate install of mplayer was required. Once installed, SMPlayer found mplayer and all is well again.