As a SPA developer I want to run the page editor on a SPA on an external server, so that developing with it is easier, maintaining it is easier, and so that authors get a perfect preview
Currently, it is not possible for the page editor to directly operate on a page loaded from a different domain in an Iframe due to browser security restrictions.
In a truly headless scenario, the SPA may be running on a external server. For example React apps often run on a Next server, Vue apps often run on a Nuxt server.
There is an Incubator module which does this: