### What
Prefetches to pages within a shared layout would frequently cache miss
despite having the data available. This causes the "instant navigation"
behavior (with the 30s/5min TTL) to not be effective on these pages.
### Why
In #59861, `nextUrl` was added as a prefetch cache key prefix to ensure
multiple interception routes that correspond to the same URL wouldn't
clash in the prefetch cache. However this causes a problem in the case
where you're navigating between sub-pages. To illustrate the issue,
consider the case where you load `/foo`. This will populate the prefetch
cache with an entry of `{foo: <PrefetchCacheNode}`. Navigating to
`/foo/bar`, with a link that prefetches back to `/foo`, will now result
in a new cache node: `{foo: <PrefetchCacheNode>, /foo/bar%/foo:
<PrefetchCacheNode>}` (where `Next-URL` is `/foo/bar`). Now we have a
cache entry for the full data, as well as a cache entry for a partial
prefetch up to the nearest loading boundary. Now when we navigate back
to `/foo`, the router will see that it's missing data, and need to
lazy-fetch the data triggering the loading boundary.
This was especially noticeable in the case where you have a route group
with it's own loading.js file because it creates a level of hierarchy in
the React tree, and suspending on the data fetch would result in the
group's loading boundary to be triggered. In the non-route group
scenario, there's still a bug here but it would stall on the data fetch
rather than triggering a boundary.
### How
In #61794 we conditionally send `Next-URL` as part of the `Vary` header
if we detect it could be intercepted. We use this information when
creating the prefetch entry to prefix it, in case it corresponds with an
intercepted route.
Closes NEXT-2193
b9861fd2cd