After a hard work, I figured out the problem.
For example,
- one of my contact sends me an emotion sound while his window is open here.
- I click on "Play XXX sound"
- Then while listening the sound I click on chat box once but don't move my mouse, I mean I leave the curser where I click.
- Then I can't write any letter in the chat box. (I have to _move_ mouse a bit (not necessarily click once more))
If you still can't reproduce it, just keep working on the next version as usual, forget about this problem
/ 
