comment form causing strange css behavior

On my site (https://www.mountyscorner.com/blog) I’m seeing some strange behavior. The main blog index.php page looks fine, but when IE users click into a page controlled by single.php, the grey bar to the left of the post jumps about 20 px to the left. This behavior isn’t happening in any other browser on any other platform that I’ve tested – just all versions of IE in Windows. Before you ask, both the HTML and CSS validate.

When I started playing in comments.php, I found that if I set the cols property of the comment text area to 60, the following would happen:

1. the grey bar would end up in the right position
2. the textarea would sit right at the edge of the commentarea div
3. there would be a patch of blank space under the “allowed tags” section before the textarea showed up

If I set the cols to 59, everything would line up properly vertically, but the grey bar would shift to the left.

Interestingly enough, those bubbles on the sidebar seem to determine how much whitespace shows up (the [+/-] collapses and expands each individual bubble). So if I collapse enough of them, everything again lines up just right, with the grey bar in the correct position.

So I’m guessing that the sidebar (floated right) is exacerbating the problem. I tried setting the width of the comment_area – I got to 69% before the grey bar started shifting again. Apparently something is overlapping with that sidebar? Whatever the problem is, I can’t find it, and I’ve spent a few hours digging through all my CSS and included files. Can one of you masters take a fresh look at it and figure out what’s happening?