In order to perform the drag and drop functionality in Page Builder in browser tests we are loading the drag-mock library:
https://github.com/ezsystems/ezplatform-admin-ui/blob/1.5/src/lib/Behat/Helper/UtilityContext.php#L342

Instead of doing it during every page load we could speed things up, adding the js to Page Builder Twig template using Webpack Encore (ConfigManager add method - https://doc.ezplatform.com/en/latest/guide/bundles/#configuration-from-a-bundle)