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.
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