Kihwan Jung 

Tricky one... so some guesses to try and be helpful.

• Try changing the font to ensure you are using a fully unicode compliant font.  Some fonts may automatically replace half-width characters with their full-width counterparts if the original font lacks support for the requested character. 

• If you're typing in Korean and accidentally switch to a mode that defaults to full-width characters, this might cause sudden changes.  This might be triggered by pressing a hotkey unintentionally.

• Some IMEs attempt to "normalise" text to match expected character widths, especially in mixed-script text.  Perhaps this is at play here?

Hi,
It has been happening to me as well on Hungarian-Korean translations.
In certain fields (a few % of the whole project) the input switches to full-width mode. The previous and following field is normal.
I guess it is not a font issue as when I copy it to Word and change to any font, it remains full-width. Also I didn't change font between doing the previous, the problematic and the following field.
Mixed script is a possibility, as there are latin characters in those fields, but so are in many others where this problem doesn't happen.

Hi,
It has been happening to me as well on Hungarian-Korean translations.
In certain fields (a few % of the whole project) the input switches to full-width mode. The previous and following field is normal.
I guess it is not a font issue as when I copy it to Word and change to any font, it remains full-width. Also I didn't change font between doing the previous, the problematic and the following field.
Mixed script is a possibility, as there are latin characters in those fields, but so are in many others where this problem doesn't happen.

Kihwan Jung 

If not yet done, please go to File -> Options -> Editor -> Formatting display style and change it to "Show all tags but do not show formatting". This is helpful with font issues in my language, so it might help you too.

Thanks, but it is not font issue, it is a Unicode issue