I created a presentation with LibreOffice 6 (under Windows), and tried to view the slideshow, but the slides appear split on the screen. The same presentation works fine using LibreOffice 5.4.1 (in Ubuntu). Is anyone using LibreOffice 6, and do they have similar issues?
Is Impress starting with slides at 16:9 ratio and your screen is 4:3? Does machine with Ubuntu same machine where you have Windows?