As colorbox uses javascript, a page that does not validate could respond erratically. When I used an online html validation tool for the above provided product page, there was an area that had the text 'cart.</p> </p>'. The second paragraph closure appears to not have an equivalent opener.
Additionally, I haven't been able (yet) to use a real computer to evaluate the load process to see if there may be a javascript error occurring, but considering that the goal and effect is to hide a portion of the screen, the existing html validation issues appear to relate.
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