Just need to handle few more messages in editing area:
http://msdn.microsoft.com/en-us/library/windows/desktop/dd318168(v=vs.85).aspx
Please fix it.
I also report similar(same?) issue on MacOSX lion.
Kana to Kanji conversion is interrupted when hitting tab key.It's reproduced on Google Japanese Input, and probably also on Kotoeri and Atok which I don't confirm..