To reproduce:
1. Open attached Unity project "Bug.zip"
2. Open "Campaign Scene" scene
3. Go into Play Mode
4. Move the mouse to the Game View Window

Expected result: The Console should keep on printing "Mouse is NOT over Game Object" message since mouse is not hovering over any UI element
Actual result: The Console starts printing "Mouse is over Game Object" message when the mouse is on the Game View Window despite that the UI panel is on a different display (see attached gif "MultiDisplayBug.gif")
Note: This bug is not reproducible in the build

Reproduced on: 5.6.0b2, 5.6.1p3, 2017.1.0b9, 2017.2.0a3
Note: On 5.6.0b1 and older versions the IsPointerOverGameObject function does not seem to work at all (UI panel does not get recognized even on the correct display) so there is no way to know if the bug is present in those versions