I started playing this with a steam controller. When playing with that controller you could hit the R/L back trigger (under bumper? not the trigger and not the bumper) and cycle the active row of inventory. Convenient way to switch between a weapon and a pick for instance.
Now I'm on a new computer and trying to bind the steam controller correctly. Or set up a brand new "just right for me" controller config.
I can't figure out what key actually does this. It's not on the wiki and I can't find it in any steam discussions.
Best Answer
Turns out that it is not possible on keyboard (bug?) and can only be configured using a controller. There is a mod on Nexus which allows you to do this by using (by default) the tab key. The key is configurable!
Here is an image of how it works (from the mod page):