fixes NEXT-2173
Fixes#59968
### TODOs
- [x] recreate [repro](https://github.com/williamli/nextjs-NEXT-2173)
- [x] patch `useSelectedLayoutSegment` to support parallel routes (see
"What")
- [x] check `useSelectedLayoutSegments` to see if it is affected
- [x] add test cases
- [x] finalise PR description
### What?
`useSelectedLayoutSegment` does not return the name of the active state
of parallel route slots.
#### Expected Behaviour
According to
https://nextjs.org/docs/app/building-your-application/routing/parallel-routes#useselectedlayoutsegments
> When a user navigates to app/@auth/login (or /login in the URL bar),
loginSegments will be equal to the string "login".
👉🏽 We should update the docs to explain `null` and __DEFAULT__ result as
well.
According to the [API reference for
useSelectedLayoutSegment](https://nextjs.org/docs/app/api-reference/functions/use-selected-layout-segment#returns):
> useSelectedLayoutSegment returns a string of the active segment or
null if one doesn't exist.
> For example, given the Layouts and URLs below, the returned segment
would be:
> <img width="881" alt="CleanShot 2024-01-20 at 14 50 52@2x"
src="https://github.com/vercel/next.js/assets/179761/bfaa34c8-3139-4ec3-bd70-4346c682e36b">
#### Current Behaviour
Currently a string "children" is returned for everything inside a
parallel route with active state and `__DEFAULT__` is returned if there
is no active state for the parallel route (since the `default.tsx` is
loaded). ~`null` is returned when the `default.tsx` is not loaded
(possibly caused by another bug, see test case 5).~
#### Reproduction
[GitHub Repo](https://github.com/williamli/nextjs-NEXT-2173) is created
based on the example provided in [Next.js docs for using
`useSelectedLayoutSegment` with Parallel
Routes](https://nextjs.org/docs/app/building-your-application/routing/parallel-routes#useselectedlayoutsegments).
#### Test Cases
1. If you visit https://next-2173.vercel.app/, you get loginSegments:
__DEFAULT__ (hard navigation) or children (soft navigation after
returning from a visit to /login)
2. If you soft nav to (/app/@auth/login and /app/@nav/login)
https://next-2173.vercel.app/login, you get
1. loginSegment: `children` (expected value should be `login`)
2. navSegment: `children` (expected value should be `login`)
3. If you soft nav to (/app/@auth/reset)
https://next-2173.vercel.app/reset, you get
1. loginSegments: `children` (expected value should be `reset`)
2. navSegment: `children` (expected value should be `login`)
4. If you soft nav to (/app/@auth/reset/withEmail)
https://next-2173.vercel.app/reset/withEmail, you get
1. loginSegments: `children` (expected value should be `withEmail`)
2. navSegment: `children` (expected value should be `login`)
5. ~If you hard nav to (/app/@auth/reset/withEmail)
https://next-2173.vercel.app/reset/withEmail, you get an unexpected
result due to possibly another bug:~
* ~navSegment is `null` on the deployed (Vercel) version, the navSlot is
*not* loaded~
* ~navSegment is `__DEFAULT__` on local dev, the navSlot loads
`/app/@nav/default.tsx`.~
### Why?
In `packages/next/src/client/components/navigation.ts`,
`getSelectedLayoutSegmentPath` is called and returns the correct
segmentPath for parallel routes (even though there is a TODO comment
indicating this function needs to be updated to handle parallel routes)
but `useSelectedLayoutSegment` failed to return the correct segment when
a parallelRouteKey is provided.
### How?
`useSelectedLayoutSegment` is updated to return
selectedLayoutSegments[0] for non parallel routes (original logic), but
it will return the last segments for parallel routes (or null if nothing
is active).
```
return parallelRouteKey === 'children'
? selectedLayoutSegments[0]
: selectedLayoutSegments[selectedLayoutSegments.length-1] ?? null
```
---------
Co-authored-by: Zack Tanner <zacktanner@gmail.com>
78c9793a53