Theme JSON: replace top-level background style objects on merge

[ramonjd]

What?

This PR fixes an omission in the theme json merge logic where top-level background image objects are not replaced, rather they are merged, which was the state of affairs before #64128

Blocks are already catered for via ::get_block_nodes().

Why?

Consider the following: I want to merge existing JSON A with incoming JSON B.

{
	"title" : "JSON A",
	"styles": {
		"background": {
			"backgroundImage": {
				"id": 1,
				"url": "http://example-site/wp-content/uploads/11/2024/test.jpg"
			}
		}
	}
}

{
	"title" : "JSON B",
	"styles": {
		"background": {
			"backgroundImage": {
				"url": "http://some-other-site/new.jpg"
			}
		}
	}
}

In trunk the outcome would be:

{
	"title" : "JSON A",
	"styles": {
		"background": {
			"backgroundImage": {
				"id": 1,
				"url": "http://some-other-site/new.jpg"
			}
		}
	}
}

Notice the "id" property, which does not belong to the new image. The entire object should be replaced.

How?

Add the top-level styles path to the paths to be merged.

Testing Instructions

Run the unit tests: npm run test:unit:php:base -- --filter WP_Theme_JSON_Gutenberg_Test

[github-actions]

Flaky tests detected in 72a47a3.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11623192696
📝 Reported issues:

• [Flaky Test] Top level navigation links inherit the text color from the theme/group but do not apply to the submenu or overlay text #53977 in /test/e2e/specs/editor/blocks/navigation-colors.spec.js

[github-actions]

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Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

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[andrewserong]

Good catch! Yeah, I could imagine the merge being unexpected if and when we allow the UI to support folks using external urls for background images, too.

The logic looks good to me, and I gave background images in theme.json and uploaded via the site editor a smoke test with a variety of permutations and didn't run into any issues.

LGTM! 🚀

[ramonjd]

Thanks for testing!!

[aaronrobertshaw]

Bit slow to the party (as usual) but this works as advertised for me also. All the merge variations I tried produced the expected results.

Thanks for the fix!