Since the playlist button is on the left I was wondering it it wouldn't make more sense to have the playlist on the left. This would also have the advantage that we could use the entire height for content, since there's nothing above it there. That way we'd avoid the divider entirely, since the entire sidebar can scoll. On mobile/narrow it would be a flap rather than a full-width view, so as to always have a window drag handle somewhere.
The sidebar background would be a darkened/lightened version of the background gradient, ignore the somewhat random colors here.