### What?
When navigating between pages (via `prefetch: false`) within a dynamic
segment, the shared layout is re-rendered. This can cause unexpected
behavior like layout data changing when navigating between child
segments.
### Why?
When prefetch is false, we're currently opting into an "optimistic
navigation" codepath, which will optimistically render layout-routers up
to the point where data is missing, while kicking off data fetches. It
attempts to determine where refetching needs to happen by traversing the
router cache nodes and checking where data is missing. However, it
locates these cache nodes by using "segments" obtained by
[deconstructing the
URL](https://github.com/vercel/next.js/blob/fix/optimistic-bailout/packages/next/src/client/components/router-reducer/reducers/navigate-reducer.ts#L142),
which won't accurately contain dynamic segment data. For ex, `/en` which
corresponds with `/app/[lang]/page.tsx` will have a cache node key of
`lang|en|d`, not `en`. Similarly, the optimistic tree that gets
constructed will also be incorrect, since it uses the URL segment.
### How?
My initial fix was to match the dynamic segment against the segment
constructed by the URL. But after discussion with @sebmarkbage and the
team, it seems more correct to remove the optimistic case all together
as there's no guarantee that the url will actually match to that
segment.
Fixes#50670
---------
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
Zack Tanner