WYSIWYG Editor: Vertical scrollbar missing in IE8

When editing a XWiki Page in the WYSIWYG-Editor and the page is longer than the size of the editing frame, a vertical scrollbar would be helpful, but is not displayed in IE 8. No such problem in IE 9.

To reproduce, simply go to the edit view of Sandbox.WebHome
The scrollbar is missing, even though one can "scroll" around e.g. by moving the cursor around with the "arrows" keys.

Tested with IE 8.0.6001.18702, Windows XP Service Pack 3 Build 2600

This problem is caused by a

overflow: visible !important;

in xwiki-platform-core/xwiki-platform-wysiwyg/xwiki-platform-wysiwyg-client/src/main/resources/org/xwiki/gwt/wysiwyg/public/stylesheets/RichTextArea.css, line 9
Changing this to

overflow: auto !important;

fixes the problem for me.

However I am not sure why this css rule is there and what it is supposed to fix.