Under "Manage Jenkins" -> "Configure System":
Using MSIE 7 or 8, the "System Message" panel shows no border, is much larger than necessary, and the resize handle below it is non-functional (though cursor changes to up+down arrows)..
Using Firefox or Chrome (don't know version) or MSIE 9 (I think), the panel shows a border and is sized correctly, but the resize handle is non-functional (though cursor changes to up+down arrows).
Other resizable panels (eg: Extended E-Mail Notification's "Default Content") work correctly.