el nino,
I did the debugging for you.
The problem is with your settings for the text entry form for the "Add Comment" section.
You have the column width set to 120 columns. I don't know a whole lot about HTML but knowing what I know about other programming languages, there should be a way to fix the width to stay within the confines of a certain area regardless of font or screen resolution.
Anyway, I set the columns to 100 from 120 on this line:
<textarea name="Comment" class="expand_small_input" rows="3" cols="100" style="overflow: auto;"></textarea>
And it fixed the entire problem.
The problem could be caused by Safari using a larger-point fixed-width font because I have my screen resolution set pretty high. It could also be why the problem didn't manifest on your computer; you may be using a lower screen resolution than me so Safari in turn uses a smaller-point font.
That is also why the problem doesn't show up if you are not logged in-- there is no "Add Comment" text box f*cking up the formatting.