https://www.shrivenhamheritagesociety.co.uk/src/popup.js

1function toggle(div_id) {
2	var el = document.getElementById(div_id);
3	if ( el.style.display == 'none' ) {	el.style.display = 'block';}
4	else {el.style.display = 'none';}
5}
6function blanket_size(popUpDivVar) {
7	if (typeof window.innerWidth != 'undefined') {
8		viewportheight = window.innerHeight;
9	} else {
10		viewportheight = document.documentElement.clientHeight;
11	}
12	if ((viewportheight > document.body.parentNode.scrollHeight) && (viewportheight > document.body.parentNode.clientHeight)) {
13		blanket_height = viewportheight;
14	} else {
15		if (document.body.parentNode.clientHeight > document.body.parentNode.scrollHeight) {
16			blanket_height = document.body.parentNode.clientHeight;
17		} else {
18			blanket_height = document.body.parentNode.scrollHeight;
19		}
20	}
21	var blanket = document.getElementById('blanket');
22	blanket.style.height = blanket_height + 'px';
23	var popUpDiv = document.getElementById(popUpDivVar);
24	popUpDiv_height=40;//150 is half popup's height
25	popUpDiv.style.top = popUpDiv_height + 'px';
26}
27function window_pos(popUpDivVar) {
28	if (typeof window.innerWidth != 'undefined') {
29		viewportwidth = window.innerHeight;
30	} else {
31		viewportwidth = document.documentElement.clientHeight;
32	}
33	if ((viewportwidth > document.body.parentNode.scrollWidth) && (viewportwidth > document.body.parentNode.clientWidth)) {
34		window_width = viewportwidth;
35	} else {
36		if (document.body.parentNode.clientWidth > document.body.parentNode.scrollWidth) {
37			window_width = document.body.parentNode.clientWidth;
38		} else {
39			window_width = document.body.parentNode.scrollWidth;
40		}
41	}
42	var popUpDiv = document.getElementById(popUpDivVar);
43	window_width=window_width/2-430;//150 is half popup's width
44	popUpDiv.style.left = window_width + 'px';
45}
46function popup(windowname) {
47	blanket_size(windowname);
48	window_pos(windowname);
49	toggle('blanket');
50	toggle(windowname);		
51}