Created attachment 545671 [details]
See the screenshot. I think what's happening is that we take the height of the document, but we don't account for the height of the web console area. In other words, if web console is H pixels tall, we think that the browser window is H pixels shorter than it really is.
Yes, that sounds about right. We recently changed the position of the html panel because of some test failures.
This should be fixed when we complete: https://bugzilla.mozilla.org/show_bug.cgi?id=672002
no longer applicable. Thank you for your bug, Ehsan!