`sveltekit:prefetch` potentially loads a wrong page when an anchor link is involved

[ivanhofer]

Describe the bug

When using a link with sveltekit:prefetch that contains an anchor-link e.g. '/blog#modal' the page gets requested on hover, which is the intended behaviour.
But if you then decide to not click on that link and visit the same page without the anchor-link /blog you will see the contents of the link you hovered before.

In this implementation the onMount hook accesses the $page store which holds a wrong value.

I would assume when using prefetch sveltekit only loads the necessary load functions and components.
The $page store should get updated when you actually click on the link.

Reproduction

Here is the link to a Repo showing the issue: https://stackblitz.com/edit/sveltejs-kit-template-default-uubxoo

1. load the homepage
2. hover over one of the last two links
3. click the first link

You should see that the page was loaded with some content that belongs to the other links

Logs

No response

System Info

System:
    OS: Windows 10 10.0.22000
    CPU: (12) x64 Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz
    Memory: 16.90 GB / 31.73 GB
  Binaries:
    Node: 16.13.2 - ~\AppData\Local\Volta\tools\image\node\16.13.2\node.EXE
    Yarn: 1.22.17 - ~\AppData\Local\Volta\tools\image\yarn\1.22.17\bin\yarn.CMD
    npm: 8.1.2 - ~\AppData\Local\Volta\tools\image\node\16.13.2\npm.CMD
  Browsers:
    Edge: Spartan (44.22000.120.0), Chromium (100.0.1185.50), ChromiumDev (99.0.1141.0)
    Internet Explorer: 11.0.22000.120
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.37 
    @sveltejs/kit: next => 1.0.0-next.320 
    svelte: ^3.46.0 => 3.47.0

Severity

serious, but I can work around it

Additional Information

No response

[elliott-with-the-longest-name-on-github]

@Rich-Harris

Created #4773 as this seems extremely straightforward. Feel free to toss it or provide feedback if the solution doesn't cover all of the edge cases -- though I think it's fine!