Here's how I fixed IntelliJ IDEA opening to a grey/null screen while running under bspwm or other tiling WM's. According to this section in bspwm's page on the Arch Wiki, it's an issue with the way some Java programs respond to tiling WM's. There are several fixes and pages listed there; below is what I found to work best, YMMV.
- jdk11-openjdk & wmname via pacman
sudo pacman -S jdk11-openjdk wmname
- Switch system to jdk11 (this fixed crashes I was getting with jdk16, might not be necessary)
archlinux-java jdk11-openjdk