I noticed this in online game of simutrans-extended, but I think that the chat code is unmodified so this should apply to both versions.
When I send or receive a message it pops-up in dialog window and steals the focus. Further keystrokes then are interpreted as commands, potentially doing unwanted stuff. I tried to set chat messages to not pop-up, but only run in the ticker, but they still pop-up.
So first bug - they pop-up all the time,
Second bug - would it be possible to make those windows not getting focus?