Steps to reproduce:

Note: Need keyboard with Korean language setup(windows 10 Microsoft IME Korean will be fine)

1) Download attached project "Korean repro.zip" and open in Unity.
2) Open scene "test" and press Play.
3) Change keyboard language in Korean and type some characters in the Input Field.
4) Highlight the whole text and type some more Korean characters(see attached "highlight.gif").
5) You should receive Error message in Console tab "ArgumentOutOfRangeException: Cannot be negative."

Reproduced with : 5.3.5p5, 5.4.0b19