Once a label is dragged to a different label track, it gets stuck in place

1. Have two label tracks
2. Add a label of any type
3. Start dragging it across the same label track
4. Observe: it moves freely
5. Now, drag it onto another label track and try to move it sideways
6. Observe: the clip gets stuck in place and cannot be moved either left or right.

Start by reproducing the drag sequence with two label tracks: move a label within its original track, then move it to the other track and try to move it sideways. Trace the label-track drag handling from that interaction, and consider the issue done when labels remain horizontally movable after being moved between tracks.

Written by the indexing model from the issue text.