         
         return removeNavigationItem(at: _navigationItems.index(before: _navigationItems.endIndex))
     }
+    
+    // MARK: Navigation Title Item
     
     public func addNavigationTitleItem(_ item: TabNavigationTitleItem) {
         _addNavigationTitleItemButton(item)

        return _removeNavigationTitleItemButton(at: _navigationTitleItems.index(before: _navigationTitleItems.endIndex))
    }
    
    public func addNavigationTitleActionItem(_ item: TabNavigationTitleActionItem) {
        addNavigationTitleItem(item)
    }

}

extension TabNavigationBar {
    // MARK: - Public Property.
    
    /// Get the selected title item of the tab-navigation bar.
    public var selectedTitleItem: TabNavigationTitleItem? {
        guard _earlyCheckingIndex(_selectedTitleItemIndex, in: _navigationTitleItems) else {
