Nope, I don't have a problem with that... Screenshot
I don't see anything that would obviously cause this, so I'd suggest you take Firebug (a Firefox plugin) and try to find out which element causes this bug. Usually I'd do it for you, but it's quite hard to do when I can't reproduce the bug
No, that's normal, your content is wider than the viewport, that's why I (deliberately) made the screen smaller (to make a scrollbar appear). It doesn't show much empty space, like in your screenshot. Maybe the scrollbar style confuses you, but it's scrolled all way to the right. There's nothing more to the right of it, and just the content to the left of it. Whereas in your screenshot, there's an awful lot of blue on the right of the content, and you've not even scrolled all way to the right yet.
Oh, you're right. It's 'coz it happens only with logged in users. I guess it has got something to do with the quick reply box.
Fortunately, I succeeded in hiding it by addin 'overflow-x: hidden;' into the body's style, though I still don't know what is "overflowing". Does it matter anyway....?
I'll make a test account later tonight to see it then (I'm a bit short on time atm.)
Overflow is what happens when the contents of an element are bigger than the size of that element (like a bucket in which you poor too much water). Overflow-x is overflow on the x axis (horizontal), value 'hidden' means that you just don't show whatever is too big for the element.
Overflow-x should only work when the width of an object given though, so I suggest you add "width: 100%;" to the body's style as well, to avoid future problems.
OK the quick reply box is there
Part 1: bug confirmed.
Part 2: confirmed the bug lies with the quick reply box.
Now for fixing it.
You've got an (unnamed) div that contains the quick reply box (the one below <a name="quickreply"></a>). Its width is set to 200%. That's what causes the overflow. When I set it to 100% the overflow is gone, but the quick reply box only fills half of the place. However, the first child of this div (this is a div of class "tborder") has its width set to 50%. Change that to 100% and the quick reply box has its normal size, while the overflow is still gone.
Edit: and just to specify, you can now get rid of the overflow-x. The bug doesn't occur as long as the overflow is set as well.