### What?
While scrolled on a page, and when following a link to a new page and
clicking the browser back button or using `router.back()`, the scroll
position would sometimes restore scroll to the incorrect spot (in the
case of the test added in this PR, it'd scroll you back to the top of
the list)
### Why?
The refactor in #56497 changed the way router actions are processed:
specifically, all actions were assumed to be async, even if they could
be handled synchronously. For most actions this is fine, as most are
currently async. However, `ACTION_RESTORE` (triggered when the
`popstate` event occurs) isn't async, and introducing a small amount of
delay in the handling of this action can cause the browser to not
properly restore the scroll position
### How?
This special-cases `ACTION_RESTORE` to synchronously process the action
and call `setState` when it's received, rather than creating a promise.
To consistently reproduce this behavior, I added an option to our
browser interface that'll allow us to programmatically trigger a CPU
slowdown.
h/t to @alvarlagerlof for isolating the offending commit and sharing a
minimal reproduction.
Closes NEXT-1819
Likely addresses #58899 but the reproduction was too complex to verify.
a578cc8192