To reproduce:
1. Open project attached
2. In project window, double-click the Animator controller AnimC to open it
3. In Animator window, try right-clicking anywhere on Animator grid

Expected: menu is opened at current cursor's location
Actual: menu is opened in various locations

Reproduced in: 2017.3.0a1
Not reproduced in: 5.4.5p4, 5.5.4p2, 5.6.2p4, 2017.1.0p1, 2017.2.0b4
Regression starts in: 2017.3.0a1

Note: menu's location is dependent on viewport's position in Animator grid (position can be adjusted by holding mouse wheel and dragging)