Cover: Merge block and global styles

[Mamaduka]

What?

Fixes #49384.

PR adds missing dimension styles when using a Cover block as a featured image with a fixed background.

Why?

The render function was replacing the already generated styles attribute.

How?

I've updated the logic to merge styles before re-applying them.

Testing Instructions

1. Open a Post or Page.
2. Insert a Cover Block.
3. Set the image for the cover block to the Featured image.
4. In Dimensions, add margin, padding, and a min-height.
5. In Media settings, toggle the Fixed background on.
6. Confirm all inline styles are applied correctly when previewing the post.

Screenshots or screencast

[Mamaduka]

The min-height is already serialized in the content. So there's no need to add it separately now that the style attribute values are merged.

P.S. We might implement this via the dimensions support flag. See #45300.

[github-actions]

Flaky tests detected in b8278ba.
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/4553455959
📝 Reported issues:

• [Flaky Test] Keeps focus in place on undo in template parts #49386 in /test/e2e/specs/site-editor/template-part.spec.js

[t-hamano]

Thank you for the PR!

This PR will fix padding and margin as well as other global styles to render correctly when using a Cover block as a featured image with a fixed background. 🎉

Before

After

[t-hamano]

Suggested change

	$styles = $processor->get_attribute( 'style' );
	$merged_styles = ! empty( $styles ) ? $styles . ';' : '';
	$merged_styles .= 'background-image:url(' . esc_url( $current_featured_image ) . ');';
	$styles = $processor->get_attribute( 'style' );
	$merged_styles = ! empty( $styles ) ? $styles . ';' : '';
	if ( $current_featured_image ) {
	$merged_styles .= 'background-image:url(' . esc_url( $current_featured_image ) . ');';
	}

Maybe this should be done as a follow-up PR, but how about preventing the background-image property with an empty url from being output when there is no post featured image?

[Mamaduka]

Right. Let's do this in a follow-up 👍

[Mamaduka]

Actually, there's no need for that. You can't enabled a fixed background if if the featured image isn't set. So this code is never executed.

[t-hamano]

I think the following steps could happen:

• Add the featured image to the post from the Post panel
• Insert a cover block
• Enable "Fixed background" or "Repeated background"
• Delete the featured image from the Post panel

After this procedure, the media settings will not appear in the cover block, but the hasParallax or isRepeated attributes will be retained.

[Mamaduka]

Good point. Never trust the user 😄

I've changed on a local branch and will push it shortly.