I am unsure if this comment is valid in this case: 

I think it is more common to establish the QDrag on mouseMoveEvents 
https://github.com/mixxxdj/mixxx/blob/7905a7e6454d77f635b97f94758f269ba0e911fb/src/widget/wtracktableview.cpp#L793
In this case it makes no different, but we might want to use the left click for other Actions later.