I have a ZUL with an iframe, where pdfs are loaded and displayed. I also have a draggable popup window. The problem I'm seeing is that, during the drag motion, the window appears to pass underneath the pdf. When I release the mouse button to stop dragging, the window then appears on top of the pdf. I'm using the following at the top of ZUL:

<?script content="zk.useStackup = 'auto/gecko';"?>

The popup window is mode="overlapped".

See ZKFiddle example: http://temp.zkfiddle.org:8080/t651/rddvqs/1/

Click the login link in the upper right corner of the ZKFiddle example, and the try to drag the Login window over the PDF document. The problem shows in IE9, but works correctly in Safari and Chrome on the Mac.