Line remaining in project explorer when dragging the element between other two, but then going out of bounds of the project explorer.

Reproduction steps:

1. Create a project
2. Create 3 new scripts
3. Create new folder
4. Place 2 scripts in that folder
5. Drag third script to place it between other two
6. Don't place it, but move your mouse out of bounds
(easier to do if layout is setup so that it's game view immediately left to the project explorer and it's in one column view)

Reproduced on: 2017.1.0b2, 2017.1.0b3
Not reproduced on: 5.6.0f3, 2017.1.0b4

Fixed on 2017.1.0b4