Looking for source of the widget combo button in the CKEditor Samples » Widgets page

Hi!

What you're looking for is the menubutton plugin. Take a look on this sample:

CKEDITOR.replace( 'editor1', {
    plugins: 'wysiwygarea,toolbar,basicstyles,menubutton',
    on: {
        pluginsLoaded: function() {
            // Item definitions. 
            // Let's bind them with bold and italic. For fun.
            var items = {
                bold: {
                    label: 'Drop-down bold',
                    // Let's use existing commands.
                    command: 'bold',
                    group: 'some_group',
                    icon: 'bold'
                },
                italic: {
                    label: 'Drop-down italic',
                    command: 'italic',
                    group: 'some_group',
                    icon: 'italic'
                }
            };

            // Items must belong to a group.
            this.addMenuGroup( 'some_group' );
            this.addMenuItems( items );

            var editor = this;

            this.ui.add( 'Drop-down', CKEDITOR.UI_MENUBUTTON, {
                label: 'Drop-down label',
                // Let's use an existing icon.
                icon: 'underline',
                onMenu: function() {
                    // You can control the state of your commands live, every time
                    // the menu is opened.
                    return {
                        bold: editor.commands.bold.state,
                        // Italic will always be disabled (example).
                        italic: CKEDITOR.TRISTATE_DISABLED
                    };
                }
            } );
        }
    }
} );

Also take a look on language plugin's source where you'll find another example of drop-down menu.