Relative URLs resolve incorrectly if clicked while another route is currently loading.

[dimfeld]

Describe the bug

When a route is loading and the load is taking a while, clicking another link resolves the href against the destination page's URL instead of the current page, causing the navigation to go to the wrong place.

Say you have a page with this link: <a href="subdir/post">Go to page</a>, and the load function takes a while to resolve. Clicking it once will correctly navigate to /subdir/post, but if you click the link again while it's loading the router will try to access /subdir/subdir/post and encounter a 404 error.

Reproduction

I first encountered this on my own personal website when on a very slow internet connection, but I have a minimal reproduction at https://github.com/dimfeld/sveltekit-routing-double-click

Steps:

1. This is a standard Sveltekit project. Install deps, run pnpm dev, and load the site.
2. Click the "Go to page" link. The route that this loads intentionally delays the load function by 5 seconds.
3. While it's loading, click "Go to page" again and see that it navigates to a nonexistent route.

Logs

No response

System Info

System:
    OS: macOS 12.0.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 1.37 GB / 32.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.13.0 - /opt/homebrew/Cellar/node@16/16.13.0/bin/node
    Yarn: 1.22.17 - /opt/homebrew/Cellar/node@16/16.13.0/bin/yarn
    npm: 8.1.0 - /opt/homebrew/Cellar/node@16/16.13.0/bin/npm
  Browsers:
    Brave Browser: 96.1.32.106
    Safari: 15.1
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.3
    @sveltejs/kit: next => 1.0.0-next.201
    svelte: ^3.44.0 => 3.44.2

Severity

blocking an upgrade

Additional Information

No response

[dummdidumm]

Somewhat related to #2870, although the current stance is to not wait on changing the URL, so the solution to this is then probably independent of #2870

[dimfeld]

Didn't notice this at first, but also notably, once you reproduce the problem, you end up with three history entries:

1. /
2. /subdir/post
3. /subdir/subdir/post

That feels wrong since technically the user never sees page 2. Even without this bug you would see something like this (in a scenario where the second link clicked is different from the first):

1. /
2. /subdir/post
3. /subdir/otherpost

So as part of this fix I wonder if it would be more correct to use replaceState instead of pushState if navigating during loading.

[dimfeld]

Fixed by #4070