Enable up/down keys for navigating the ULS menu

css/jquery.uls.lcd.css

@@ -138,3 +138,7 @@
 	bottom: 0;
 	left: 0;
 }
+
+.language-option--highlighted {
+	background-color: #eaeff7;
+}

src/jquery.uls.languagefilter.js

@@ -115,6 +115,14 @@
 						this.options.onSelect( query, e );
 					}
 
+					break;
+				case 38: // arrow up
+					this.options.lcd.navigateUp();
+
+					break;
+				case 40: // arrow down
+					this.options.lcd.navigateDown();
+
 					break;
 			}
 		},
@@ -163,6 +171,10 @@
 				results = [],
 				query = ( this.$element.val() || '' ).trim().toLowerCase();
 
+			// Reset the keyboard navigation index inside LanguageCategoryDisplay (lcd)
+			// before re-rendering the language options
+			this.options.lcd.resetNavigationIndex();
+
 			if ( query === '' ) {
 				this.options.lcd.setGroupByRegionOverride( null );
 				this.resultHandler( query, languages );

src/jquery.uls.lcd.js

@@ -59,6 +59,11 @@
 		this.$cachedQuicklist = null;
 		this.groupByRegionOverride = null;
 
+		// The index of the language option that is currently visited using arrow key navigation
+		// Can take values in the [0, languageOptionListItemsLength - 1] range for top to bottom navigation,
+		// or in the [-1, -languageOptionListItemsLength + 1] range for bottom to top navigation.
+		this.navigationIndex = null
+
 		this.render();
 		this.listen();
 	}
@@ -66,6 +71,74 @@
 	LanguageCategoryDisplay.prototype = {
 		constructor: LanguageCategoryDisplay,
 
+		/**
+		 * Returns a jQuery object containing a collection of all the
+		 * language option <li> elements
+		 * @return {jQuery}
+		 */
+		getLanguageOptionListItems() {
+			return this.$element.find( 'li[data-code]' );
+		},
+
+		/**
+		 * Increases the keyboard navigation index by one and applies a specific
+		 * class to the n-th language option <li> element (where n = navigation index)
+		 * Currently used as event handler for the arrow down 'keydown' event, inside
+		 * LanguageFilter.
+		 */
+		navigateDown: function () {
+			// We support navigation starting both from the top and the bottom of the language list.
+			// The navigation should stop when the last language option is already highlighted (for top to bottom navigation).
+			// For top to bottom navigation, that happens when navigation index is equal to languageOptionListItemsLength - 1.
+			// For bottom to top navigation, that happens when navigation index is equal to -1.
+			if ( this.navigationIndex === this.getLanguageOptionListItems().length - 1 || this.navigationIndex === -1 ) {
+				return;
+			}
+
+			if ( this.navigationIndex === null ) {
+				this.navigationIndex = 0
+			} else {
+				this.navigationIndex++;
+			}
+			this.highlightLanguageOption();
+		},
+
+		/**
+		 * Decreases the keyboard navigation index by one and applies a specific
+		 * class to the n-th language option <li> element (where n = navigation index)
+		 * Currently used as event handler for the arrow down 'keydown' event, inside
+		 * LanguageFilter.
+		 */
+		navigateUp: function () {
+			// We support navigation starting both from the top and the bottom of the language list.
+			// The navigation should stop when the first language option is already highlighted (for bottom to top navigation).
+			// For top to bottom navigation, that happens when navigation index is equal to 0.
+			// For bottom to top navigation, that happens when navigation index is equal to -languageOptionListItemsLength + 1.
+			if ( this.navigationIndex === 0 || this.navigationIndex === -this.getLanguageOptionListItems().length + 1 ) {
+				return;
+			}
+
+			this.navigationIndex--
+			this.highlightLanguageOption();
+		},
+
+		/**
+		 * Adds a specific class ("language-option--highlighted") only to the n-th
+		 * language option <li> element (where n = navigation index)
+		 */
+		highlightLanguageOption() {
+			this.getLanguageOptionListItems().removeClass( 'language-option--highlighted' );
+			this.getLanguageOptionListItems().eq( this.navigationIndex ).addClass( 'language-option--highlighted' );
+		},
+
+		/**
+		 * Resets the navigation index to null.
+		 * Currently used inside LanguageFilter search method, to reset the keyboard navigation
+		 */
+		resetNavigationIndex: function () {
+			this.navigationIndex = null;
+		},
+
 		/**
 		 * Adds language to the language list.
 		 *