STEPS TO REPRODUCE 1. Open several font files, each in its own kfontview window 2. Install a font from within one of the windows and wait for that to finish OBSERVED RESULT The Install button in all other kfontviewer windows becomes disabled SOFTWARE/OS VERSIONS KDE Plasma Version: 5.20.2 KDE Frameworks Version: 5.75.0 Qt Version: 5.15.1
Confirmed on KDE Neon User Edition. Installed in Personal.
This can also be observed when opening an already installed font. This disables the Install button in all other instances. Conversely, you can enable the Install button for already installed fonts by opening a non-installed font. In this case, the installation will result in an error message. See BUG 461374.