Twenty Twenty-One: Allow local anchor links to be used in primary navigation.

This adds some JavaScript to detect when an anchor link is clicked within the primary navigation on mobile devices and closes the menu before scrolling to the location on the page.

Props poena, macmanx, t-p.
Merges [49854] to the 5.6 branch.
Fixes #52006.

git-svn-id: https://develop.svn.wordpress.org/branches/5.6@49855 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-content/themes/twentytwentyone/assets/js/primary-navigation.js

@@ -150,6 +150,23 @@ function twentytwentyoneExpandSubMenu( el ) { // jshint ignore:line
 			}
 		} );
 
+		/**
+		 * Close menu and scroll to anchor when an anchor link is clicked.
+		 * Adapted from TwentyTwenty.
+		 */
+		document.addEventListener( 'click', function( event ) {
+			// If target onclick is <a> with # within the href attribute
+			if ( event.target.hash && event.target.hash.includes( '#' ) ) {
+				wrapper.classList.remove( id + '-navigation-open', 'lock-scrolling' );
+				twentytwentyoneToggleAriaExpanded( mobileButton );
+				// Wait 550 and scroll to the anchor.
+				setTimeout(function () {
+					var anchor = document.getElementById(event.target.hash.slice(1));
+					anchor.scrollIntoView();
+				}, 550);
+			}
+		} );
+
 		document.getElementById( 'site-navigation' ).querySelectorAll( '.menu-wrapper > .menu-item-has-children' ).forEach( function( li ) {
 			li.addEventListener( 'mouseenter', function() {
 				this.querySelector( '.sub-menu-toggle' ).setAttribute( 'aria-expanded', 'true' );