app-router: prefetching tweaks (#52949)
This PR tries to address some feedback around prefetching, like in #49607, where they were some warnings because we were over prefetching.
The tweaks in this PR:
- if there are no loading boundary below, we don't prefetch the full page anymore. I made that change a while ago but I think it wasn't the original intent from @sebmarkbage. Fixing that now. So now, we will prefetch the page content until the nearest loading boundary, only if there is any.
- there's now a queue for limiting the number of concurrent prefetches. This is to not ruin the bandwidth for complex pages. Thanks @alvarlagerlof for that one.
- also, if the prefetch is in the queue when navigating, it will get bumped.
- bonus: fixes a bug where we were wrongly stripping headers in dev for static pages
Test plan:
<img width="976" alt="CleanShot 2023-07-20 at 17 53 43@2x" src="https://github.com/vercel/next.js/assets/11064311/2ea34419-c002-4aea-94df-57576e3ecb2e">
In the screenshot you can see that:
- only 5 requests at a time are authorised
- when I clicked on 15, it got prioritised in the queue (did not cancel the rest)
- the prefetch only included the content until the loading boundary
Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>
