https://pac.sals.edu/polaris/scripts/ios-orientationchange-fix.js

1/*! A fix for the iOS orientationchange zoom bug.
2 Script by @scottjehl, rebound by @wilto.
3 MIT / GPLv2 License.
4*/
5(function(w){
6	
7	// This fix addresses an iOS bug, so return early if the UA claims it's something else.
8	var ua = navigator.userAgent;
9	if( !( /iPhone|iPad|iPod/.test( navigator.platform ) && /OS [1-5]_[0-9_]* like Mac OS X/i.test(ua) && ua.indexOf( "AppleWebKit" ) > -1 ) ){
10		return;
11	}
12
13    var doc = w.document;
14
15    if( !doc.querySelector ){ return; }
16
17    var meta = doc.querySelector( "meta[name=viewport]" ),
18        initialContent = meta && meta.getAttribute( "content" ),
19        disabledZoom = initialContent + ",maximum-scale=1",
20        enabledZoom = initialContent + ",maximum-scale=10",
21        enabled = true,
22		x, y, z, aig;
23
24    if( !meta ){ return; }
25
26    function restoreZoom(){
27        meta.setAttribute( "content", enabledZoom );
28        enabled = true;
29    }
30
31    function disableZoom(){
32        meta.setAttribute( "content", disabledZoom );
33        enabled = false;
34    }
35	
36    function checkTilt( e ){
37		aig = e.accelerationIncludingGravity;
38		x = Math.abs( aig.x );
39		y = Math.abs( aig.y );
40		z = Math.abs( aig.z );
41				
42		// If portrait orientation and in one of the danger zones
43        if( (!w.orientation || w.orientation === 180) && ( x > 7 || ( ( z > 6 && y < 8 || z < 8 && y > 6 ) && x > 5 ) ) ){
44			if( enabled ){
45				disableZoom();
46			}        	
47        }
48		else if( !enabled ){
49			restoreZoom();
50        }
51    }
52	
53	w.addEventListener( "orientationchange", restoreZoom, false );
54	w.addEventListener( "devicemotion", checkTilt, false );
55
56})( this );