Thanks! Note that the version of Widget Customizer merged into Core does not include the functionality which required you to add_theme_support( 'widget-customizer' ) to opt-in for live previews. The pseudo-live previewing of widgets in the preview was removed with the goal of abstracting the functionality so that any customizer control (not just widgets) can do partial preview updates (e.g. changing menus). So for now, when you make a change to a widget, it will end up doing a full refresh of the preview window as you enter the changes, but the customizer preview refreshing logic does try to make this unobtrusive.