Text editor widens the page

Technical Details

Forum version : #phpBB2
Position : Founder
Concerned browser(s) : Mozilla Firefox, Google Chrome
Screenshot of problem : https://2img.net/h/i1331.photobucket.com/albums/w592/bookmoron/Untitled_zpspehykrhl.png
Who the problem concerns : All members
When the problem appeared : After I finished the forum design.
Forum link : http://attenlon.bulgarianforum.net/

Description of problem

Hello, everyone! I'm sorry in advance if my question has been asked before, but I searched for a solution to my problem and I couldn't find one, so I have only one salvation - you.

You see, after I was done with the design of my forum and was satisfied enough to start creating the topics and everything else, I opened the signature option in my profile and noticed that when I do that, the text editor widens my page so much, that a scrollbar is needed in order to see the whole page.

If it's not understandable - this is what I mean:
Normal view of the forum (index)
View of the forum in Text Editor mode

All I want to do is to make the text editor unresizable, but no matter how hard I try, I can't do it. So please help me. I really want the forum to function at it's best.

Do you mean this only happens when you edit your signature?

That much - yes. When I start a topic, for example, it's okay. Until I pull the triangle in the lower right corner of the editor - and it widens the page again - the body of the forum doesn't act as a boundary for the editor.

Normal view

Widened

But when it comes to the Signature editor - it's widened by default, i.e. without having to pull the triangle.