I've had Manjaro running for a few days now, having switched from Debian (and Mint before that) because Debian refused to use my new motheboard's Intel HD Graphics.
I was just getting used to the Xfce DE and had the panel set up the way I wanted it, then tried to add a new app to the panel -- and then had nothing but a short blank extra-height "panel" at the top left (mine had been the default height and full width on the bottom.
I spent quite a while reconfiguring the panel, then selected a different icon for Thunderbird -- Boom! The same thing again.
How do I fix this permanently? I had added apps and changed icons before without any problem.