Its because the window has a limit on how far it can resize the width. My msn on windows xp stops at
Because it doesnt have the capabilities to go any further. It might be your OS thats telling you that you cant go any further with that line, sicne you cant even go to the line... But I noticed on your screen shot that it had stoped at
aswelll. The line happens in another program of mine, but the actual moving/resizing stops before it.