To reproduce:
1. Open any project
2. Try navigating the scene using the shift+ two non-opposite directional keys (try going diagonal)

Expected: directional keys' input is not ignored
Actual: after the Shift is began to be held down, new directional keys' events are ignored

Reproduced in: 2018.2.0a6, 2018.2.0b3
Not reproduced in: 2017.2.3f1, 2017.4.3f1, 2018.1.0f2, 2018.2.0a5

Note1: directional keys used: W,A,S,D
Note2: odd behavior examples:
pressing Shift and then directional buttons result in viewport camera not moving
pressing two directional keys and then shift works, but changing input to new directions while shift still on hold will be ignored
press directional button, then shift. Camera's moving, however, releasing directional keys and pressing them again will be ignored (while Shift still being held down)
Conclusion: After Shift is being pressed down and held, no new directional input will be recognised