fix: prevent beforeNavigate() for download links

[eltigerchino]

fixes #9611

This is a breaking change that causes the client to ignore download links instead of treating them as an external link.
This fixes the issue of the internal navigating flag being incorrectly set, leading to missing subsequent beforeNavigate() calls.

The navigating flag prevents the beforeunload event from triggering a second round of beforeNavigate() callbacks when an external link is clicked. Because the external link causes the page to unload, Kit doesn't care that the navigating flag isn't reset to false. In our download link case, the page isn't unloaded, causing subsequent navigations to not call beforeNavigate(). Therefore, we should treat download and external links differently by opting out of the same logic as external links.

It's possible for us to still call beforeNavigate for download links but downloading isn't really navigating? I'm open to this.

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[dummdidumm]

To me this is not a breaking change but a bug fix - I don't think we ever intended to have this triggered for download, nor do any user want this behavior.
PR looks good to me 👍

[eltigerchino]

Whoops, my mistake. I did wonder if anyone out there actually used it this way. I'm fine with marking it as a patch instead.

[Rich-Harris]

LGTM though i had one small question

[Rich-Harris]

is the origin check necessary or could it just be this?

Suggested change

	const download = url?.origin === location.origin && a.hasAttribute('download');
	const download = a.hasAttribute('download');

[eltigerchino]

The origin check ensures it only triggers on valid download links.
If a user (for whatever weird reason) has an invalid link such as:

<a href="https://not-the-same-origin.com/file" download>...</a>

The browser navigates to the URL instead of downloading the file. In this case, it could be useful to have beforeNavigate() run.