Sometime the last chat message does not appear when the scrollbar is visible, to reproduced:
1- Type Test <ENTER> multiple time until you see the scrollbar
2- Scroll up to the middle so you can't see the last message
3- Type any word and scroll down
Now you can't see the last typed message, instead you may see small dot, You may need to repeat step 3 more than one time.
I'm not sure if it is related to Fluent Theme, because I wasn't able to reproduce it with Crystal Theme.