Reproduction Steps:
1. Open project
2. Build and run the project.
Note: The window displays the state of each ALT, CTL, and SHIFT key.
Note: These tests are on US-mapped keyboards which do not have an Graphics Alt key.

Alt keys work as expected when used by themselves. Note that Alt-GR is tied to Alt-Right.

Problem 1: ALT-GR behaves inconsistently when CTL is pressed:
a) Press and hold ALT-RIGHT. (Note that ALT-GR also shows pressed)
b) Press and release CTL-LEFT repeatedly. (Problem: ALT-GR tracks CTL-LEFT while ALT-RIGHT is held)
c) Release Alt-Right
d) Press and hold CTL-LEFT
e) Press and hold ALT-RIGHT (ALT-R, ALT-GR both true)
f) Release CTL-LEFT (Problem: ALT-GR momentarily switches to false)

Problem 2: SHIFT keys do not track properly when both pressed
a) Press and hold SHIFT-LEFT
b) Press and release SHIFT-RIGHT repeatedly (Problem: Note that SHIFT-RIGHT stays true until SHIFT-LEFT is released)

c) Starting with all released, Press and hold SHIFT-LEFT
d) Press and hold SHIFT-RIGHT
e) Release SHIFT-LEFT (Problem: SHIFT-LEFT often stays true for a short time--up to a second--before returning to false)

(Problem 2 scenarios also occur if keys are reversed, I.e. SHIFT-RIGHT pressed first)

Reproduced on versions: Unity 5.3.7f1, 5.4.2p4, 5.5.0b10, 5.6.0a2