movement problems with modal Dialog

movement problems with modal Dialog

A while back I asked a question regarding how to stop a fixed dialog from having its movement scaled by the vertical scroll position of a page. This post seems to have disappeared, anyhow. I've been working on another project so only recently come back to this issue which is still very much extant. I believe I posted  a JING link at the time, but it seems very easy to replicate

1) create a document such that it is larger in height than the display area permits
2) stick a button to open a dialogue down at the bottom of this document, such that you have to scroll down to click it 
3) open the dialog and try and move it whilst the window is scrolled down. If you make the document long enough the effect is immediately obvious. The window doesn't move with the mouse, rather the mouse + scroll position. 


my dialogClass on creation is fixed-dialog as follows

.fixed-dialog{
  position: fixed;
  top: 50px;
  left: 50px;
}

otherwise I'm not doing anything beyond responding to the button click with the code for example

$("#DetailsDialog").dialog({modal:true, dialogClass:"fixed-dialog", width:"700px"});

I can stop it of course with a draggable:false. Which masks the bug, but its not what I really want here. Has anyone any other ideas?