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+/**
+ * The TabIndexedElement class is an attribute mixin used to add additional functionality to an
+ * element created by another class. The mixin provides a ‘tabIndex’ property, which specifies the
+ * order in which users will navigate through the focusable elements via the "tab" key.
+ *
+ *     @example
+ *     // TabIndexedElement is mixed into the ButtonWidget class
+ *     // to provide a tabIndex property.
+ *     var button1 = new OO.ui.ButtonWidget( {
+ *         label: 'fourth',
+ *         tabIndex: 4
+ *     } );
+ *     var button2 = new OO.ui.ButtonWidget( {
+ *         label: 'second',
+ *         tabIndex: 2
+ *     } );
+ *     var button3 = new OO.ui.ButtonWidget( {
+ *         label: 'third',
+ *         tabIndex: 3
+ *     } );
+ *     var button4 = new OO.ui.ButtonWidget( {
+ *         label: 'first',
+ *         tabIndex: 1
+ *     } );
+ *     $( 'body' ).append( button1.$element, button2.$element, button3.$element, button4.$element );
+ *
+ * @abstract
+ * @class
+ *
+ * @constructor
+ * @param {Object} [config] Configuration options
+ * @cfg {jQuery} [$tabIndexed] The element that should use the tabindex functionality. By default,
+ *  the functionality is applied to the element created by the class ($element). If a different element is specified, the tabindex
+ *  functionality will be applied to it instead.
+ * @cfg {number|null} [tabIndex=0] Number that specifies the element’s position in the tab-navigation
+ *  order (e.g., 1 for the first focusable element). Use 0 to use the default navigation order; use -1
+ *  to remove the element from the tab-navigation flow.
+ */
+OO.ui.TabIndexedElement = function OoUiTabIndexedElement( config ) {
+	// Configuration initialization
+	config = $.extend( { tabIndex: 0 }, config );
+
+	// Properties
+	this.$tabIndexed = null;
+	this.tabIndex = null;
+
+	// Events
+	this.connect( this, { disable: 'onDisable' } );
+
+	// Initialization
+	this.setTabIndex( config.tabIndex );
+	this.setTabIndexedElement( config.$tabIndexed || this.$element );
+};
+
+/* Setup */
+
+OO.initClass( OO.ui.TabIndexedElement );
+
+/* Methods */
+
+/**
+ * Set the element that should use the tabindex functionality.
+ *
+ * This method is used to retarget a tabindex mixin so that its functionality applies
+ * to the specified element. If an element is currently using the functionality, the mixin’s
+ * effect on that element is removed before the new element is set up.
+ *
+ * @param {jQuery} $tabIndexed Element that should use the tabindex functionality
+ * @chainable
+ */
+OO.ui.TabIndexedElement.prototype.setTabIndexedElement = function ( $tabIndexed ) {
+	var tabIndex = this.tabIndex;
+	// Remove attributes from old $tabIndexed
+	this.setTabIndex( null );
+	// Force update of new $tabIndexed
+	this.$tabIndexed = $tabIndexed;
+	this.tabIndex = tabIndex;
+	return this.updateTabIndex();
+};
+
+/**
+ * Set the value of the tabindex.
+ *
+ * @param {number|null} tabIndex Tabindex value, or `null` for no tabindex
+ * @chainable
+ */
+OO.ui.TabIndexedElement.prototype.setTabIndex = function ( tabIndex ) {
+	tabIndex = typeof tabIndex === 'number' ? tabIndex : null;
+
+	if ( this.tabIndex !== tabIndex ) {
+		this.tabIndex = tabIndex;
+		this.updateTabIndex();
+	}
+
+	return this;
+};
+
+/**
+ * Update the `tabindex` attribute, in case of changes to tab index or
+ * disabled state.
+ *
+ * @private
+ * @chainable
+ */
+OO.ui.TabIndexedElement.prototype.updateTabIndex = function () {
+	if ( this.$tabIndexed ) {
+		if ( this.tabIndex !== null ) {
+			// Do not index over disabled elements
+			this.$tabIndexed.attr( {
+				tabindex: this.isDisabled() ? -1 : this.tabIndex,
+				// ChromeVox and NVDA do not seem to inherit this from parent elements
+				'aria-disabled': this.isDisabled().toString()
+			} );
+		} else {
+			this.$tabIndexed.removeAttr( 'tabindex aria-disabled' );
+		}
+	}
+	return this;
+};
+
+/**
+ * Handle disable events.
+ *
+ * @private
+ * @param {boolean} disabled Element is disabled
+ */
+OO.ui.TabIndexedElement.prototype.onDisable = function () {
+	this.updateTabIndex();
+};
+
+/**
+ * Get the value of the tabindex.
+ *
+ * @return {number|null} Tabindex value
+ */
+OO.ui.TabIndexedElement.prototype.getTabIndex = function () {
+	return this.tabIndex;
+};