Steps to reproduce:
1. Load http://petitions.pm.gov.uk/Open-IT-projects/ on a slow-ish network connection.
2. Note that a horizontal scrollbar appears during the page load.
3. Scroll down the page.
The horizontal scrollbar remains fixed to the content and detaches from the bottom of the screen. Drag a selection through the scrollbar to notice that the area repaints correctly.
This doesn't reproduce when i reload the page after is already loaded. This appears to be due to some resources being cached so the initial layout takes place at a different time.
See also bug 8909 and a comment on why I think this is happening under bug 12440.
*** This bug has been marked as a duplicate of 11224 ***