Make charts vertically responsive (chartjs#3105) (chartjs#3326)

Ensure that the hidden iframe is stretched vertically in order to detect height changes. Remove the classlist check/call since it was incorrectly spelled (should be classList), but also useless since the iframe has just been generated. Also remove the callback check: addResizeListener should never be called w/o a valid callback.

src/core/core.helpers.js

@@ -958,38 +958,32 @@ module.exports = function(Chart) {
 	};
 	helpers.addResizeListener = function(node, callback) {
 		// Hide an iframe before the node
-		var hiddenIframe = document.createElement('iframe');
-		var hiddenIframeClass = 'chartjs-hidden-iframe';
-
-		if (hiddenIframe.classlist) {
-			// can use classlist
-			hiddenIframe.classlist.add(hiddenIframeClass);
-		} else {
-			hiddenIframe.setAttribute('class', hiddenIframeClass);
-		}
-
-		// Set the style
-		hiddenIframe.tabIndex = -1;
-		var style = hiddenIframe.style;
-		style.width = '100%';
-		style.display = 'block';
-		style.border = 0;
-		style.height = 0;
-		style.margin = 0;
-		style.position = 'absolute';
-		style.left = 0;
-		style.right = 0;
-		style.top = 0;
-		style.bottom = 0;
+		var iframe = document.createElement('iframe');
+
+		iframe.className = 'chartjs-hidden-iframe';
+		iframe.style.cssText =
+			'display:block;'+
+			'overflow:hidden;'+
+			'border:0;'+
+			'margin:0;'+
+			'top:0;'+
+			'left:0;'+
+			'bottom:0;'+
+			'right:0;'+
+			'height:100%;'+
+			'width:100%;'+
+			'position:absolute;'+
+			'pointer-events:none;'+
+			'z-index:-1;';
+
+		// Prevent the iframe to gain focus on tab.
+		// https://github.com/chartjs/Chart.js/issues/3090
+		iframe.tabIndex = -1;
 
 		// Insert the iframe so that contentWindow is available
-		node.insertBefore(hiddenIframe, node.firstChild);
+		node.insertBefore(iframe, node.firstChild);
 
-		(hiddenIframe.contentWindow || hiddenIframe).onresize = function() {
-			if (callback) {
-				return callback();
-			}
-		};
+		this.addEvent(iframe.contentWindow || iframe, 'resize', callback);
 	};
 	helpers.removeResizeListener = function(node) {
 		var hiddenIframe = node.querySelector('.chartjs-hidden-iframe');