Some browsers report the window height incorrectly differently - particularly mobile browsers, which have a different viewport concept. I sometimes use a function to check several different values, returning whichever is the greatest. For example

function documentHeight() {

    return Math.max(

        window.innerHeight,

        document.body.offsetHeight,

        document.documentElement.clientHeight

    );

}

Edit: I just looked at how jQuery does it and it does indeed use Math.max and a series of properties - however the list it checks is slightly different to those in my example above, and since I usually trust the jQuery team to be better at this stuff than I am; here is the non-jQuery jQuery solution (if that makes any sense):

function documentHeight() {

    return Math.max(

        document.documentElement.clientHeight,

        document.body.scrollHeight,

        document.documentElement.scrollHeight,

        document.body.offsetHeight,

        document.documentElement.offsetHeight

    );

}