The Issue: I can explain it better in a gif like this.
Description: It looks like the scrollbar does not maintain the hover event which expands the panel, so the other panel goes away when you mouse over the scrollbar. This happens when you want to add a track to a playlist that's nested in a folder.
How to reproduce:
Step 1: Have lots of playlists so a scroll bar shows in the window that appears when you right-click a track and hover over "Add to playlist".
Step 2: Make a folder and put some playlists in it.
Step 3: Try to add a track to the playlist you just made in the folder.
Note: If you move your mouse faster than usual over the scrollbar, you have a chance that the window will stay open (see end of the gif)
What I've tried: Contacting support on Twitter and followed up all their suggestions. Even though to me it seems like a coding issue.
Windows 10 on Desktop
Premium Spotify account
Spotify version: 22.214.171.1249.g4637b02a
Solved! Go to Solution.