Dialog box displays full width + unwanted scroll in IE6

Here's how the issue can be reproduced:

• XE 1.9 SNAPSHOT 17115 + IE6 on WinXP
• Edit a page with the new WYSIWYG editor
• Scroll until the editor menu bar nears the top of the browser viewport
• Click on a dialog-box-opening button (import office document, insert image...)
• The dialog box takes the page's full width
• The page automatically scrolls up thus the dialog box isn't centered anymore

The expected behavior is the one that works in FF3 in the same version (centered dialog box, no scroll).