How on earth was this achieved??

[jbrown1597]

https://imgur.com/a/9hfLs6c

In this screenshot from a recent 'show off your setup' thread,
OP's list of artists in the column browser is nested into menus / categories with drop-down buttons.

How on earth was this achieved?
He was on a guest account so I can't inbox him unfortunately.

[watdub]

My guess...
Nested folders are available in the Playlist section so he set up basic playlists called Rock, Pop etc.  
Then he went to his genre browser and right-clicked Rock, Pop etc and sent them to the relevant basic playlist.

[Zak]

View > Arrange Panels

Add the Library Explorer to the Left Main Panel section.

You can click the column header to select which categories to show, but note that it appears to use the Sort By fields rather than the Choose Categories command.