Select/unselect on drag depending on where the user is clicking to start the drag. <img width="1063" alt="screen shot 2018-08-16 at 16 08 06" src="https://user-images.githubusercontent.com/957103/44229492-9d293c80-a16e-11e8-9eec-242c211b154c.png">
Select/unselect on drag depending on where the user is clicking to start the drag.