Newly created group doesn't release when dragged

Steps to recreate:
1) Go to (for example) admin/structure/types/manage/article/fields.
2) Click on the Add New Group's drag icon (click once or click and drag, same effect).
4) The

does loose the "drag" class and gain "drag-previous" properly, but continues to follow the mouse up and down in the table.

This happens in both Firefox 3.6 and Chrome (both on Linux). It doesn't happen with Add New Field, so it's module specific, nor does it happen with a preexisting field group that's dragged.

I'll try to find the cause of this odd behavior in the morning - it looks like mouseUp isn't being bound properly to stop it dragging somehow.