cursor jumps to top of editor when user clicks left margin

One of our users reported an issue today where the cursor jumps to the top of the editor window if someone clicks in the area between the left edge of the editor and the start of the text on any line. This issue only seems to occur in Internet Explorer.

I can reproduce this problem on our site even in a totally clean demo, but it doesn't occur the same way in the ckeditor.com demo.

In our demo (http://www.westminster.edu/common/coding/ckeditor/_samples/replacebyclass.html), the arrow-style mouse cursor appears when the mouse is held over this margin. However, at http://ckeditor.com/demo, the text-style mouse cursor appears in this situation. Clicking in the margin on the ckeditor.com demo briefly moves the cursor to the top for about a second before moving it back down to the previous location.

Any ideas? I'm running a fresh install of CKEditor 3.5. Thanks!