### What
When a global not found page is rendered, and when the not-found page or
containing layout has a link with `prefetch: auto` back to the root
page, the router would update the URL but not correctly swap out the
not-found component with the page component.
### Why
With auto prefetching (which is the default when `prefetch` is left
unspecified on a link), the router will perform a partial prefetch on
dynamic pages. This means it'll fetch the flight data _without_ React
nodes and store it in the prefetch cache. On navigation, this is used to
determine where we already have cached React nodes and where we need to
trigger a lazy fetch to get new data. However, global not found pages
are peculiar in that they will always contain a data path like: `['', {
children: ['__PAGE__', {}] }]` since they are inserted at the root. This
means that if there's also a page component that corresponds with the
same path, the router will incorrectly think it already has cache node
data for it.
### How
During SSR when the `asNotFound` flag signals to the renderer that the
component we're rendering is matching the global not-found page, we
modify the segment key to be something unique so the data path won't
collide with a top-level page.
In
[fc01c8e](fc01c8e7f7)
I added handling only on the server to modify the segment key. This
still fixes the issue, but at the cost of triggering an MPA navigation
on the client because it's treated as a root layout change
In
[69d5687](69d5687765)
I added client handling to not treat this special segment key as a root
layout change, and to signal to the router it needs to refetch the data.
This ensures we don't do an MPA navigation.
Fixes#61956
Closes NEXT-2481
---------
Co-authored-by: JJ Kasper <jj@jjsweb.site>
fffa4c3d9b