Thanks for the post, Sarah!

If the Widget Customizer is added to core, WordPress theme developers will need to indicate that their widget areas support live previews in the customizer.

Just to clarify this: themes and widgets are not required to indicate support for Widget Customizer: all existing themes and widgets should be compatible. However, for such themes and widgets which do not indicate support for Widget Customizer, any changes made to widgets will trigger the preview window to refresh, as opposed to a more responsive “live” preview. But this is the same requirement imposed on all themes that leverage the customizer: they have to explicitly indicate support for live previews of settings changes (e.g. transport=postMessage) because every theme is different in how it uses the settings. In the same way, all widgets are different and so we cannot assume they can be updated in place without a page refresh unless they explicitly indicate they support doing so.