Steps to reproduce:
1. Open attached project "947312.zip"
2. Open "Bug Sample" scene
3. Enter play mode
4. Begin dragging image with Left Mouse Button
5. In addition, click and hold Right Mouse Button (which also starts dragging object)
6. While holding down RMB, release LMB

Actual result: OnEndDrag() gets called even though RMB is held down
Expected result: OnEndDrag() doesn't get called if there are any other active buttons dragging that object

7. While holding down RMB move mouse
8. OnDragBegin() gets called

Reproducible with: 5.6.3p2, 2017.1.1f1, 2017.2.0b10, 2017.3.0a7

Watch attached gif *image resets its position to the center of the screen on OnEndDrag()*