Following a prefetched link will fail if switching quickly back and forth

[ehrencrona]

Describe the bug
If, starting from a page A you click on a link to a page B and then - before B loads - hovering over a preloading link to A, then you will not be able to follow the link to A once B finishes loading. The URL will update, but the previous page will remain active.

To Reproduce
In https://github.com/ehrencrona/sapper-prefetch-caching-repro is the Sapper template but with a preload for the home page and for the blog page that takes 2 s to execute.

Go to /blog and reload. Then in quick succession click home, blog and home again. Wait two seconds. Now click blog. The URL will update but you will still be on the home page.

Expected behavior
I expect the blog to render.

Severity
Moderate; occurs on real public sites when preloading is slow.