Mixer/instruments windows' draggable border is hidden.

[Canaanabolaanan]

I'm pretty sure this was left over from before the playback toolbar was added.

[moppius]

In my opinion these two windows should be entirely draggable/dockable/snappable. On a widescreen monitor, the placement of these windows wastes space because the tablature is long and tall, so this window layout ends up eating vertical space while the left and right sides of the tablature are useless white space. Qt supports dockable windows so I will look into enabling this behaviour.