Introduce sticky position block support

[andrewserong]

In the Gutenberg plugin, a position block support feature was introduced last year, that allows a Group block to be set to a "sticky" position, meaning that when the page scrolls, the block will stick to the top of the window.

It would be good to backport this feature for the WP 6.2 release. While the feature is spread across JS and PHP code, it should be possible to land most of the PHP implementation in advance of the JS packages update for core.

The original feature landed in Gutenberg across the following PRs:

• ​https://github.com/WordPress/gutenberg/pull/46142
• ​https://github.com/WordPress/gutenberg/pull/47626
• ​https://github.com/WordPress/gutenberg/pull/47665

This PR backports the PHP parts of the new sticky position block support, and is a follow-on from ​https://github.com/WordPress/wordpress-develop/pull/3865

Backports the PHP parts of the following PRs:

• ​https://github.com/WordPress/gutenberg/pull/46142
• ​https://github.com/WordPress/gutenberg/pull/47626
• ​https://github.com/WordPress/gutenberg/pull/47665

Notes on this backport:

• The backport deviates from Gutenberg trunk in one way: fixed position type is excluded from the appearance tools opt-in: this is because fixed positioning isn't being opted-in for the Group block and isn't ready to be used as a default opt-in. The code paths exist for fixed positioning as it will be supported in the future, but for now, sticky is the only one exposed by default. (The overall block support should still support it as a valid value, though)
• The styling output while logged in depends on the JS packages update, as it will include the CSS changes in #47665, so while testing this PR you might notice an odd styling issue while logged in. This should be resolved once the package update lands.
• With the above caveat shouldn't really be an issue though, as there is no way for a user to set a group block to sticky without the JS packages landing. This PR is to land the required PHP changes in advance of that.

Screenshots:

| While logged in (CSS variable that's included in JS packages update isn't yet output in this PR, once it is the sticky block will sit below the admin bar) | While not logged in (correct) |
| --- | --- |
| | |

Testing instructions:

• In the Group block's block.json file add a "position": { "sticky": true } object under "supports" (Note: for this change to propagate in your testing, you might need to rebuild the block JSON cache locally).
• Create a post and add a bunch of paragraphs.
• Insert the below markup at the beginning of the post (this is Group block markup with the appropriate position style object to sticky to top).
• Publish the post, and on the site frontend, the group block should stick to the top of the screen once it has been scrolled past the top of the screen (the block sticks to the scrollable area of its container)

Group block markup for testing purposes:

{{{html

<div class="wp-block-group has-background" style="background-color:#f2aeae;padding-top:1em;padding-right:1em;padding-bottom:1em;padding-left:1em">
<p>A paragraph in a sticky group</p>
</div>

}}}

Trac ticket: https://core.trac.wordpress.org/ticket/57618

[flixos90]

I already reviewed ​https://github.com/WordPress/wordpress-develop/pull/3973 (almost good to go from my perspective), and I'm happy to commit this once it's ready.

Thanks for reviewing @felixarntz!

Does this have to wait for the JS package update, or can it be committed to core before already?

This can be committed before the JS package update, since this PR doesn't opt any of the blocks into the new control, so it doesn't expose anything to a user. I think it's preferable to land this PR first, so that when the JS packages update happens, the PHP parts will already be in place.

[hellofromTonya]

As @flixos90 has approved for commit and @andrewserong confirmed it can be committed prior to package updates, marking for commit.

[davidbaumwald]

In 55285:

Editor: Introduce sticky position block support.

In the Gutenberg plugin, a position block support feature was introduced last year, that allows a Group block to be set to a "sticky" position, meaning that when the page scrolls, the block will stick to the top of the window.

This change merges the "sticky" position feature for blocks introduced in Gutenberg 15.0.

Props andrewserong, flixos90, mukesh27.
Fixes #57618.

Merged into core in https://core.trac.wordpress.org/changeset/55285.

[davidbaumwald]

In 55286:

Editor: svn add new files missed in [55285].

Follow-up to [55285].

See #57618.