If you are chatting with another person and you have received a message, the notification window does not flash.In version 2.8.3 it works.
In 2.9.0 flashing only starts when chat window is not active. Try to click on some other window (even if chat window is still completely visible) and then receive a message. Then it flashes. I think this is better than to flash all the time.
Yes you are right.
It really flashing if you switch to another windows, thanks!