I recently installed elementary OS Hera on a laptop that previously had KDE neon. A new issue has cropped up since changing distributions - there is a strange offset on external display for presentations.
When I connect the laptop to an external display/projector using HDMI, the screen is correctly added - I can see my desktop's background on it perfectly fine.
The problem starts when I start a LibreOffice Impress presentation (slide show) - the display "window" showing the slide on the external display (while my laptop screen correctly shows the current and next slide) is offset slightly downwards from the top (where I can see a small strip of my desktop background). This chops off the bottom of the slide on display to the audience.
Since the same hardware worked without problems with LibreOffice Impress in KDE Neon (which is also based on Ubuntu 18.04 LTS), the issue is neither with the hardware, nor with underlying Linux userland, nor with LibreOffice. The only thing that changed was the distribution (and the DE - switched from KDE to Pantheon, which has a lot of GTK stuff).
Since I make multiple presentations a week, this is getting to being more than a little vexing. elementary OS configuration screens are delightfully minimalistic and functional (reminiscent of Mac OSX that I was forced to migrate from), but have not yielded anything that seems relevant.