Did you know that with the 3.9 WordPress release, the Widget Customizer Plugin was merged with WordPress core as part of its Feature-as-Plugin group?
What is the Widget Customizer?
In a nutshell, the original Widget Customizer Plugin added functionality to your Theme customizer that allowed you to experiment with your registered sidebar areas in the customizer window. The benefit of this functionality is that you are able to preview changes to your widgets in the sidebars of your site prior to committing them. This mitigates the risk of site visitors encountering a broken widget while you’re making changes or experimenting with the widget contents.
As of WordPress 3.9 Widget Customizer is baked right into WP and you can use this functionality right out of the box. If you had the plugin loaded on your site prior to WordPress 3.9 the update conveniently disabled the plugin for you.
So what kind of powers does this give you?
Registered sidebar areas appear along with the other options in the customize preview window. Selecting a widget area in the customize window opens a side menu with all of your widget options (all the options you would encounter on the admin side of your widget panel). You can add and delete widgets and preview how they will look without having to commit.
A nice benefit: if you delete a widget in the customize panel it is not permanently deleted but added to the “inactive widget” section of the widget panel.
If and when you are satisfied with your widget customizations simply “Save and Publish” and the widgets are live!
WordPress 3.9 ushered in the Feature-as-Plugin model, adding convenience and functionality to WP. With additions like the Widget Customizer built right in, we’re given a streamlined way of experimenting with and customizing our WordPress sites without risk of breaking the user experience for our site visitors.