This goes hand-in-hand with #47. In order to preview sites that use a lot of JS and Ajax requests, the requests need to be Customzier-aware. We can accomplish this via jQuery Ajax prefilter. If on the frontend, we only need to inject the customzie_snapshot_uuid query param. In the Customizer Preview, we'll need to rewrite the requests to be POST and ensure the Customzier query vars are included: this has been demonstrated to work for the WP REST API, but it will not work in all cases. See implementation from Customize REST Resources which can be lifted: https://github.com/xwp/wp-customize-rest-resources/blob/84405f4312e22e956e86b6d17f0791d64e8de372/js/rest-resources-manager.js#L208-L265