Dragging to Playlists panel shows wrong drag/drop markers that don't disappear

[sjohannes]

1. Bring up the Playlists panel.
2. Make sure you have empty space at the bottom of the treeview (collapse the list entries if needed).
3. Drag a track to the Playlists panel.
   • A green box (assuming Adwaita) appears on the empty area and doesn't disappear even after the drag finishes.

Additionally,

1. Drag towards the empty area (which now has a green border), don't release yet.
   • A blue line appears underneath the Custom Playlists entry. This does not happen when you drag anywhere else within the treeview.
2. Press escape.
   • The blue line persists.