Steps to reproduce:

1) Open Unity and create a new project
2) Select any gameObject in the Hierarchy tab(camera or Directional Light)
3) In the Inspector tab, enter any number in Position value of X field and hit Enter
Note: Input field stays highlighted and you still can put a value

Expected result: After Enter key is pressed, it should not allow to enter any number unless you hit Enter again (check attached video "expected.mp4")
Actual result: Input field stays highlighted and you still can put a value (check attached video "actual.mp4")

Not reproduced with:
5.6.4p2, 2017.1.2p1, 2017.2.0f3

Reproduced with:
2017.1.2p2 ,2017.2.2p2, 2017.3.0b1, 2017.3.0b8, 2018.1.0a3

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Fixed in:
2017.3.0p1, 2018.1.0b1