Here again visual explanation for another problem with your chat (VB ver. 3.8.1, Chat ver. 2.3.0)
1. I just type and read messages, and this was happened!?!?
2. If I reply, this problem persist!
3. Others replys also have some problem (you can see that I must scroll down slidebar to see a full message)
4. This time I just click on any other custom channel, and then return to General channel, and this problem NOW is corrected!
BUT, sometimes this procedure not helps 
Sometimes, I need to refresh page, sometimes I must close browser,...... I still cannot find reason for this problem..... I just type and read messages.... and
sometimes this problem shows.....
I think you have your vBulletin for testing, and maybee you can set input field
bottom and
older first, and test your chat for a long time.... Maybee, try to test it from time to time couple hours (do not close browser)..... Maybee you'll see this problem?!
I just want to tell you that your chat
version 1.5 don't have this problem with this settings!!!!