proxy:busy and proxy:idle whenever it is loading or is done loading content over HTTP.up.proxy.defaults('busyDelay') to delay the proxy:busy event in order to prevent flickering of loading spinners.
    
      If you're upgrading from an older Unpoly version you should load unpoly-migrate.js
      to polyfill deprecated APIs.
    
    
      Changes handled by unpoly-migrate.js are not considered breaking changes.
    
  
See our upgrading guide for details.