Are you using the website by your cell - phone, maybe ?
Most of the time such problems appear while surfing here ( being online in this site ) after hearing about several complaints about the same compalin showing again and again while being on by using phones and tablets. Some other people complained about the same problem in the past, if my memory isn't misleading me as always :D
I agree that this is a bugging problem, but as most of the problem it has a very simple solution.
Simply go to the chatter and see the words written under it , there you will find these words : " * Toggle full screen or webcam chat " .
To make the chat window dissapear and not bug you again, press on of the underlined words only once.
After you do that, the page will refresh itself but instead of seeing the same window with the chat - you will be at SoM but won't see the chat window.
And ta da , your problem is solved and the chat window disappeared..
After doing that you may keep being on the site and enjoying using it without having bugging problems with the chat window. It will dissapear.
Note : this method does not work only with people who surf here by the phone, it also works in the regular website for people who are here by using computers. So if the problem did not show while using the phone you can use this method anywhere.
So even if one who has computer is bugged by this problem - following these instructions will make the chat window dissapear and get rid of the problem. It is a solution for everyone.
I hope it helps and that this will make the problem to dissapear.