This PR implements new cache semantics for the app router on the client.
## Context
Currently, on the App Router, every Link navigation is prefetched and
kept forever in the cache. This means that once you visit it, you will
always see the same version of the page for the duration of your
navigation.
## This PR
This PR introduces new semantics for how the App Router will cache
during navigations. Here's a TL;DR of the changes:
- all navigations (prefetched/unprefetched) are cached for a maximum of
30s from the time it was last accessed or created (in this order).
- in addition to this, the App Router will cache differently depending
on the `prefetch` prop passed to a `<Link>` component:
- `prefetch={undefined}`/default behaviour:
- the router will prefetch the full page for static pages/partially for
dynamic pages
- if accessed within 30s, it will use the cache
- after that, if accessed within 5 mins, it will re-fetch and suspend
below the nearest loading.js
- after those 5 mins, it will re-fetch the full content (with a new
loading.js boundary)
- `prefetch={false}`:
- the router will not prefetch anything
- if accessed within 30s again, it will re-use the page
- after that, it will re-fetch fully
- `prefetch={true}`
- this will prefetch the full content of your page, dynamic or static
- if accessed within 5 mins, it will re-use the page
## Follow ups
- we may add another API to control the cache TTL at the page level
- a way to opt-in for prefetch on hover even with prefetch={false}
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