To assist with the development and testing of the new partial
prerendering (PPR) paradigm, this introduces a stop-gap solution to let
us verify issues with pages in preview and production environments if
enabled. When a Next.js app is built and ran with the
`__NEXT_EXPERIMENTAL_STATIC_SHELL_DEBUGGING=1` environment variable,
pages that have PPR enabled in production and preview environments can
have only their static shell served when accessed with a
`?__nextppronly=1` query parameter.
If your project is not using PPR, it will not change anything. If a page
is accessed in production or development with the query parameter but
PPR is not enabled, it will not change anything. Tests have been added
to validate that going forward.
Wyatt Johnson