I noticed that when I enter full-screen mode in Mathematica 9 (F12 for Windows) using a magnification of 125% (or any other number higher than 100%) there is a horizontal scroll bar at the bottom of the screen taking up space, provided that the original windowed notebook had lines long enough to warrant a horizontal scrollbar. This wouldn't be particularly annoying if it weren't for the fact that it's doesn't actually seem to do anything at all; clicking on it moves the scroll bar around, but nothing actually happens onscreen because fullscreen mode automatically wraps everything to the page. For some reason, it still thinks it needs a scrollbar, however, but it doesn't actually work.
I suspect this is some kind of glitch/oversight for the frontend? Here's a screenshot (small preview below):
A simple workaround is to collapse all code into their corresponding orange code blocks onscreen at the time, change magnification to the preferred value, enter fullscreen, and then expand all orange code blocks (at which point the horizontal scrollbar will not load). However, I feel like this kind of editing gymnastics shouldn't be necessary in the first place. If anyone can confirm or deny that this is a frontend visual bug, I'd be glad to hear about it.