Hello again ,
Have you tried going into your display settings of your monitor and changing the resolution? This seems to be a global problem and not just within the editor itself. Perhaps your scale or ratio is not correct, or the native resolution is not right?
Let me know. Also within the editor there are some console commands to force fullscreen mode. Press the command line prompt (~) and type in "r.Setres DesiredResolutionxDesiredResolutionf
The “f” at the end means fullscreen.
Hope this helps,