To reproduce the issue follow the steps:
1) Open attached project
2) Select "Slenderman" fbx model
3) Go to animation tab of the inspector
4) Select a clip and try to edit start or stop input fields
5) Field doesn't show that you can type numbers there.
However, you can type numbers in the field anyway.

Expected result: After pressing on input field it should appear active
Current result: Input fields does not appear to be active, however you can still type numbers into them

Video Repro:
http://screencast.com/t/xYBJxUeB

Regression from 5.3

Reproduced in 5.4.0b5 (3ef12c3e0f69), 5.4.0b4 (a7d7a3a2cef4)
Not Reproducible in 5.3.2p2 (dec88fd2a56c)