I'm facing the same issue.
my page is fairly long and whenever user opens modal panel from bottom section of the screen the modal panel seems to go slightly towards upper section and this causes to lose the visibility of the bottom section(and has to scroll down after closing the panel window).
From attributes section I can see left, top and zindex might help me in positioning the panel window.
But is there any alternative solution for this?
Here is the solution I found...
function popupRichModal(element, modalWindow)
var pos = findPos(element);
besides passing directly to JS you could define top and left using attributes.