the attached example shows the issue, that the menupopup is moving up a few pixels after the onOpen ajax event returns
the first 2 <menupopup>s have an onOpen event listener (the first one with a delay of 500ms to highlight its timing relation to the ajax response)
the 3rd <menupopup> does not have an onOpen event listener, and does not have the issue initially, only in case one of the other popups fires an onOpen (open=false) event when hovering it.
also see this screencast http://screencast.com/t/Fyv510sbLYu
The issue can be reproduced in all major recent browsers: IE11/FF/Chrome (the issue was also reported to happen in IE9)