The problem is that the first element with a relative position up on a DOM tree is a .submitUnit and since we don't have a left or right value that object element sticks to the wrong position, covering 'Create thread' button. Here is how it looks:
To solve this we just need to set position: relative for #AttachmentUploader.
Yes, Chrome. I guess that is a FlashBlock extension that breaks it, but I can't see how. It only adds a display: block !important; to the <object> element.
And yes it blocks the button, I can't push it so I have to remove this node in DevTools.