Enhancement
The rendering of the button bar and the buttons in this component should take place via HTML templates, ThemeVars.
Code migration
The AbstractButtonBarRenderer class has been removed. The DefaultButtonBarRenderer now inherits from AbstractControlRenderer instead of AbstractButtonBarRenderer. In future, the controls will be rendered via the templates, which is why the methods from DefaultControlRenderer are not required. If this is still desired or needed, the effect can be achieved by deriving DefaultButtonBarRenderer from DefaultControlRenderer<ButtonBarControl>.
Test
Start the application and check whether the buttons in the button bar and the button bar itself work exactly the same as before the refactoring.