There seems to be at least 2 systems in place here…
1. The method mentioned by turingtest2, clicking that 'Add To' button in the top right of the UI, which shifts the entire UI & lets you add to the current playlist by browsing files on the left area.
2. Drag a file without the sidebar active (it seems to work in most views) the right part of the screen slides in with a full list of playlists & folders to drop files into.
Mode 1: 'Right sidebar mode' doesn't show up if you select a playlist folder or smart playlist or any tracks within those containers?
Also it doesn't appear if you are viewing Songs or Albums or anything that isn't a playlist?
So my tracks already need to be in a 'normal' playlist so I can add it to a playlist via this method?
How is this any better than the 'right click > Add to Playlist' context menu? Wrong sidebar mode seems more appropriate.
Mode 2: Is more usable & makes sence that it doesn't show up when the sidebar is active, since the playlists are already visible, but usually scrolled offscreen, it's nice that the smart playlists are filtered out, condensing the length of the list.