I was playing around with the drop-and-drag support when I noticed the dashed outline for the dragged item was not at the point I expected, but several hundred pixels to the left!
The pixel calculation seems to be based from the edge of the nearest containing dom element, and not from the edge of the window.
In my case the form element is in a table that has fixed width and is centred on the page, so there is a lot of space either side of the containing table when the browser width is expanded. If I shrink the browser window to the same size as the containing table layout, everything works as expected. This seems to be a problem in Firefox 2, as I tried in Opera and it seems to calculate the pixel offset correctly.
Is there anything I can provide for the Jira case?