Theme Json: Don't output double selectors for elements inside blocks

[scruffian]

What?

When elements are used inside blocks, if the element selector is the same as the block selector (as it is in the case of heading blocks) then the selector becomes doubled (e.g. h1 h1). To ensure we can use element declarations inside block definitions we need to ensure that when the selectors match we don't double them. Fixes #39698.

Why?

To make it possible to use element selectors inside heading block rules.

How?

We detect whether the element selector matches the block selector. If it does, then we should only use one.

Testing Instructions

1. Add this to your block theme:

		"blocks": {
			"core/heading": {
				"color": {
					"text": "blue"
				},
				"elements": {
					"h1": {
						"color": {
							"text": "purple"
						}
					}
				}
			}
		},

2. Add a post with multiple heading blocks.
3. Check that the h1 headings are purple, but the others are still blue.

Screenshots or screencast

cc @WordPress/block-themers

[draganescu]

I tested and it works as described. The code changes makes sense to me. Caveat I am not sure of other potential implications.

[scruffian]

I'm going to merge this one but keep an eye on it in case there are unexpected side effects.

[ntsekouras]

Why do we need this here instead of naming WP_Theme_JSON_Gutenberg the class in 6.1? Is there a specific reason?

[scruffian]

The reason is that when we come to add more compat versions of this file we need to change the class name to the specific version and add a new one to the next compat folder which can create conflicts. Keeping the WP_Theme_JSON_Gutenberg in experimental is more stable and predictable.

[ntsekouras]

I'm not sure I see any difference in having WP_Theme_JSON_Gutenberg extends WP_Theme_JSON_6_0 in the 6.1 folder. I think experimental is the folder for changes that are not planned for the next WP release - thus the name. If we know something is going to be in 6.1 for example we should add it just there. --cc @gziolo

[gziolo]

I guess it's how things are done for this particular class that gets updated in every WordPress major release.