Fix the problem that scroll position can't restored at page refresh after PJAX enabled.

[gapplef]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Issue resolved:
The scroll position can't restored at page refresh after PJAX enabled.
For reference, see MoOx/pjax#214

What is the new behavior?

• Link to demo site with this changes:
• Screenshots with this changes:

How to use?

In NexT _config.yml:

[welcome]

Thanks so much for opening your first PR here!

[CLAassistant]

All committers have signed the CLA.

[stevenjoezhang]

Thanks for your contribution!

[stevenjoezhang]

The cause of the problem is that the following code was not executed when the page was first loaded
https://github.com/MoOx/pjax/blob/480334b18253c721ba648675e90261f948e2bca0/index.js#L287-L289

The scrollRestoration option is confusing. See also MoOx/pjax#111

[welcome]

Congrats on merging your first pull request here! 🎉 How awesome!

[PaperStrike]

Browser's own scroll restoration has a drawback that the restoration usually happens before we can fetch to update our HTML structure, which is bad for our Pjax sites. For example, in a Pjax site, page A at a scroll position of 200, page B has a height of 80, we navigate from A to B and navigate back, then we have page A at a scroll position of 80. (You can try it by visiting our theme-next.js.org. Consider it as the page A, scroll down a lot, enter an article as page B and navigate back.)
Things go worse when the fetch failed as the scroll position changed without content updated.

I believe that's the reason why Pjax developed its own restoration.

My solution to this is setting history.scrollRestoration back to auto before page reload instead of setting scrollRestoration to false in Pjax option. It fixes all these problems. Like:

window.addEventListener('beforeunload', () => {
  history.scrollRestoration = 'auto';
});

Code above should be in next-theme/pjax if accepted.