Twenty Seventeen: Adds `background-attachment: fixed;` to devices that should support it

iOS  disables this feature under the hood, but it also distorts the images – unlike other mobile devices that don't support it. So this adds a  check for both `background-attachment: fixed` support or whether it’s an iOS device - passing it adds the class `background-fixed` which is used to add the proper styles.

It also lowers the media query so the parallax-like style is present on a wider range of screens since this bug can be better targeted and avoided. In this way, screens that aren't the offending devices aren't punished merely based on screen size.

Props joemcgill, laurelfulford, helen.

Fixes #38395.

Built from https://develop.svn.wordpress.org/trunk@39297


git-svn-id: http://core.svn.wordpress.org/trunk@39237 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentyseventeen/assets/js/global.js

@@ -148,6 +148,34 @@
 		return 'http://www.w3.org/2000/svg' === ( 'undefined' !== typeof SVGRect && div.firstChild && div.firstChild.namespaceURI );
 	}
 
+	/**
+	 * Test if an iOS device.
+	*/
+	function checkiOS() {
+		return /iPad|iPhone|iPod/.test(navigator.userAgent) && ! window.MSStream;
+	}
+
+	/*
+	 * Test if background-attachment: fixed is supported.
+	 * @link http://stackoverflow.com/questions/14115080/detect-support-for-background-attachment-fixed
+	 */
+	function supportsFixedBackground() {
+		var el = document.createElement('div'),
+			isSupported;
+
+		try {
+			if ( ! ( 'backgroundAttachment' in el.style ) || checkiOS() ) {
+				return false;
+			}
+			el.style.backgroundAttachment = 'fixed';
+			isSupported = ( 'fixed' === el.style.backgroundAttachment );
+			return isSupported;
+		}
+		catch (e) {
+			return false;
+		}
+	}
+
 	// Fire on document ready.
 	$( document ).ready( function() {
 
@@ -184,6 +212,10 @@
 		if ( true === supportsInlineSVG() ) {
 			document.documentElement.className = document.documentElement.className.replace( /(\s*)no-svg(\s*)/, '$1svg$2' );
 		}
+
+		if ( true === supportsFixedBackground() ) {
+			document.documentElement.className += ' background-fixed';
+		}
 	});
 
 	// If navigation menu is present on page, adjust it on scroll and screen resize.

wp-content/themes/twentyseventeen/style.css

@@ -3603,6 +3603,11 @@ object {
 		max-height: 1200px;
 	}
 
+	/* With panel images 100% of the screen height, we're going to fix the background image where supported to create a parallax-like effect. */
+	.background-fixed .panel-image {
+		background-attachment: fixed;
+	}
+
 	.page-two-column .panel-content .entry-header {
 		float: left;
 		width: 36%;
@@ -3993,13 +3998,6 @@ object {
 	}
 }
 
-@media screen and ( min-width: 85.45em ) {
-
-	.panel-image {
-		background-attachment: fixed;
-	}
-}
-
 @media screen and ( max-width: 48.875em ) and ( min-width: 48em ) {
 
 	.admin-bar .site-navigation-fixed.navigation-top,

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-beta4-39296';
+$wp_version = '4.7-beta4-39297';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.