I'm on Ubuntu 15.04 with Firefox 39.0+build5-0ubuntu0.15.04.1, and with my window maximized the <body> alternates between 1411x689px and 1404x689px (with the difference being related to the size of the scrollbars that keep (dis)appearing.
I can see the style attributes of some <div>s are being manually updated by JavaScript, probably reacting to a window resize event caused by the (dis)appearing scrollbars changing the viewport size (which probably triggers the contents to change, which triggers showing/hiding the scrollbars...).
