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If you look at the top two posts in the image below, you should be able to see that just above where it says "Show post links", there's the standard single boundary line between the posting area and the space where the links can be shown. But in the bottom post, there are two boundary lines instead of one. I've tested and have found both lines remain if you click "Show post links". That looks very strange and inconsistent. Do you know what could possibly be causing that?

I guess I'll try a clean install and see if I can figure out what's causing it.

I never found out the reason for this and it's still a problem. Can anyone suggest an answer?