I see the problem with Safari 5.1 (Snow Leopard) but not with Safari 6.2 (Mountain Lion). Of course, one has to be logged into moodle.org for this div to display at all.

I inspected the CSS of the div in question and do not see anything obviously wrong at the first glance. There is something, however, that makes this div block display very, very tall instead of a single line. I actually see two different states of that. One as shown above, where the checkbox is stretched up to like 1,5 inch. First, though, I usually see as attached here, like 4 inches high. The checkbox then transforms into smaller stretch the above but the div stays tall (on fast connection the interim state might not be visible).