Scroll down a little, drag image to page
The dropupload anchor position is shifted by the browser scroll position
Drop upload anchor position matches the anchor
Dropupload _dragShow() function finds anchor position using
ofs = $anchor.offset()
Which includes the scroll position of the browser into the offset.
However, the dropupload sets the the CSS position:static property on the component, so it doesn't need to also be offset by browser scroll position
Do not scroll browser, instead have a window containing the whole UI with the data-scrollable attribute:
alternatively, use JS workaround in attached fiddle.
(JS workaround is overriding a lot of functions, so only use if necessary)