When deploying to Vercel, preview deployment URLs are dynamically
generated. This breaks Live Preview within those deployments because
there is no mechanism by which we can detect and set that URL within
Payload. Although Vercel provides various environment variables at our
disposal, they provide no concrete identifier for exactly _which_ URL is
being currently previewed (you an access the same deployment from a
number of different URLs).
The fix is to support _relative_ live preview URLs, that way Payload can
prepend the application's top-level domain dynamically at render-time in
order to create a fully qualified URL. So when you visit a Vercel
preview deployment, for example, that deployment's unique URL is used to
load the iframe of the preview window, instead of the application's
root/production domain. Note: this does not fix multi-tenancy
single-domain setups, as those still require a static top-level domain
for each tenant.
Deprecates `getPayloadHMR` and simplifies this pattern into a single
`import { getPayload } from 'payload'`.
We will still retain the exported `getPayloadHMR` but it now will throw
a deprecation warning with instructions for how to migrate.
Live preview e2e tests had no CSS when tested against prod.
For all our other tests, we have a separate test/app directory that
imports CSS. Otherwise, the root-level /app directory is used.
For live-preview, we currently always run against test/live-preview/app,
that has no CSS import.
This PR adds a new test/live-preview/prod/app directory that imports CSS
and is used when we run tests against prod.
In order for this to work, I had to make import map generation smarter
