### What?
There are a bunch of different bugs caused by the same underlying issue,
but the common thread is that performing any sort of router cache update
(either through `router.refresh()`, `revalidatePath()`, or `redirect()`)
inside of a parallel route would break the router preventing subsequent
actions, and not resolve any pending state such as from `useFormState`.
### Why?
`applyPatch` is responsible for taking an update response from the
server and merging it into the client router cache. However, there's
specific bailout logic to skip over applying the patch to a
`__DEFAULT__` segment (which corresponds with a `default.tsx` page).
When the router detects a cache node that is expected to be rendered on
the page but contains no data, the router will trigger a lazy fetch to
retrieve the data that's expected to be there
([ref](5adacb6912/packages/next/src/client/components/layout-router.tsx (L359-L370)))
and then update the router cache once the data resolves
([ref](5adacb6912/packages/next/src/client/components/layout-router.tsx (L399-L404))).
This is causing the router to get stuck in a loop: it'll fetch the data
for the cache node, send the data to the router reducer to merge it into
the existing cache nodes, skip merging that data in for `__DEFAULT__`
segments, and repeat.
### How?
We currently assign `__DEFAULT__` to have `notFound()` behavior when
there isn't a `default.tsx` component for a particular segment. This
makes it so that when loading a page that renders a slot without slot
content / a `default`, it 404s. But when performing a client-side
navigation, the intended behavior is different: we keep whatever was in
the `default` slots place, until the user refreshes the page, which
would then 404.
However, this logic is incorrect when triggering any of the above
mentioned cache node revalidation strategies: if we always skip applying
to the `__DEFAULT__` segment, slots will never properly handle reducer
actions that rely on making changes to their cache nodes.
This splits these different `applyPatch` functions: one that will apply
to the full tree, and another that'll apply to everything except the
default segments with the existing bailout condition.
Fixes#54173Fixes#58772Fixes#54723Fixes#57665
Closes NEXT-1706
Closes NEXT-1815
Closes NEXT-1812
