Prevent adding adding AMP hooks for Infinite Scroll on legacy AMP post templates

[westonruter]

Fixes a regression introduced in #16048. See #16048 (comment):

This appears to not be working properly. AMP validation errors are being emitted on this AMP page, for example: https://alankent.me/2020/06/12/musings-about-5g-and-ar/amp/

The problem seems to be twofold:

1. The <amp-next-page> element is erroneously getting added when in legacy Reader mode.
2. The amp-next-page script is not getting injected because output buffer is being used to inject the tag and/or it's not getting detected during validation because WordPress.com is still on 1.4.x and in that old version the page is not sent through the post-processor. See Pass entire Reader mode templates through post-processor ampproject/amp-wp#3781.

The validation errors currently occurring on AMP pages on WordPress.com using the Reader mode template when infinite scroll is enabled:

Changes proposed in this Pull Request:

• Prevent initializing AMP infinite scroll on legacy Reader post templates.

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

1. Active AMP plugin
2. Set the template mode to "Reader"
3. Select the AMP Legacy reader theme
4. Enable Infinite Scroll in Jetpack,
5. View an AMP page.
6. See validation errors (on AMP plugin v1.4.x) or else unexpected amp-next-page (on AMP plugin v1.5+).

Proposed changelog entry for your changes:

• Fix infinite scroll fro erroneously being added on AMP Reader mode post templates.

[jetpackbot]

Scheduled Jetpack release: October 6, 2020.
Scheduled code freeze: September 29, 2020

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17175

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Generated by 🚫 dangerJS against 4d0627a

[jeherve]

Thanks for the report! @pereirinha Do you think you could take a look? Thanks!

[jeherve]

This tests well on my end.

[jeherve]

Internal reference: D49673-code -- r213721-wpcom

[westonruter]

@jeherve I just found that this also causes a problem with Web Stories. It is causing the <amp-next-page> element to be injected on pages that should only have <amp-story>. I'm going to amend this PR with that fix.

[westonruter]

Done. When viewing a singular Web Story:

Before

…
</amp-story><amp-next-page max-pages="0">
	<script type="application/json">
		[
			{"title":"","url":"","image":""}		]
	</script>
	<div separator>
		<hr class="post-separator styled-separator is-style-wide section-inner" aria-hidden="true">
		</div>
	<div recommendation-box class="recommendation-box">
		<template type="amp-mustache">
			{{#pages}}
			<div id="infinite-handle" class="read-more-button-wrap">
	<span>
		<a href="{{url}}" class="more-link" rel="amphtml">
			<span class="faux-button">
				Older posts			</span>
		</a>
	</span>
</div>
				{{/pages}}
		</template>
	</div>
	<div footer>
		
	</div>
</amp-next-page>
		</body></html>

After

…
</amp-story></body></html>

cc @swissspidy

[jeherve]

Still tests well for me, nice work! 👍

[jeherve]

r213776-wpcom

[pereirinha]

Thank you @westonruter and @jeherve.
It is also working fine for me in the mentioned scenarios.