Herbsluftwm does not natively come with support for a 'proper' floating layer. We can remedy that problem using some scripting; the goal is to get a 'seamless' experience.
First you need to download and run the floatmon script somewhere in autostart
.
Using this is a good start.
But if you use multiple monitors you will run into the issue where if you focus right,
sometimes you can focus an empty floatmon monitor.
So probably you want to disable the focus_crosses_monitor_boundaries
option.