Fullscreen is disabled for pages whose rendered content height is less than the minimum viewport height (i.e. when both toolbars are completely visible).  For pages whose content height is greater than the maximum viewport height (i.e. when toolbars are completely hidden), fullscreen works normally and toolbars are can be completely scrolled away.  However, for pages whose content height is between these values, attempting to scroll towards the bottom of the page produces some jankiness.