https://www.arthaey.com/bin/trackViewportSize.js

1// From jQuery: http://dev.jquery.com/browser/trunk/jquery/src/dimensions.js
2function getWindowDimensions() {
3    var elem = window;
4    if (!elem) return null;
5
6    var dim = new Object();
7    var width, height;
8
9    // does it walk and quack like a window?
10    if ("scrollTo" in elem && elem.document) {
11        // Everyone else use document.documentElement or document.body depending on Quirks vs Standards mode
12        if (elem.document.compatMode === "CSS1Compat") {
13            width  = elem.document.documentElement["clientWidth"];
14            height = elem.document.documentElement["clientHeight"];
15        }
16        else {
17            width  = elem.document.body["clientWidth"];
18            height = elem.document.body["clientHeight"];
19        }
20    }
21    // is it a document?
22    else if (elem.nodeType === 9) {
23        // Either scroll[Width/Height] or offset[Width/Height], whichever is greater
24        width = Math.max(
25            elem.documentElement["clientWidth"],
26            elem.body["scrollWidth"], elem.documentElement["scrollWidth"],
27            elem.body["offsetWidth"], elem.documentElement["offsetWidth"]
28        );
29        height = Math.max(
30            elem.documentElement["clientHeight"],
31            elem.body["scrollHeight"], elem.documentElement["scrollHeight"],
32            elem.body["offsetHeight"], elem.documentElement["offsetHeight"]
33        );
34    }
35
36    dim["width"]  = width;
37    dim["height"] = height;
38    return dim;
39}