Media: Enhance logic to determine LCP image in block themes and avoid lazy-loading it.

[52065] originally introduced the logic to guess the LCP image based on certain heuristics and to not lazy-load that image. However, with the introduction of block themes, that logic was not functioning correctly, resulting in all featured images to be lazy-loaded, regardless of whether it was the LCP image or not.

Together with an update to the `core/post-featured-image` block included in [55079], this changeset fixes the logic to correctly handle featured images in block themes as well.

Additionally, in combination with an update to the `core/template-part` block from [55246], this changeset includes an enhancement which uses the benefits of block template parts to avoid lazy-loading images in the `header` block template part, making the lazy-loading heuristics even more accurate for sites using a block theme.

Props flixos90, adamsilverstein, mamaduka, antonvlasenko, shahidul95, reduanmasud, costdev, mukesh27, ironprogrammer, manfcarlo, robinwpdeveloper, spacedmonkey.
Fixes #56930.


git-svn-id: https://develop.svn.wordpress.org/trunk@55318 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-template.php

@@ -241,7 +241,7 @@ function get_the_block_template_html() {
 	$content = wptexturize( $content );
 	$content = convert_smilies( $content );
 	$content = shortcode_unautop( $content );
-	$content = wp_filter_content_tags( $content );
+	$content = wp_filter_content_tags( $content, 'template' );
 	$content = do_shortcode( $content );
 	$content = str_replace( ']]>', ']]>', $content );
 

src/wp-includes/media.php

@@ -5444,25 +5444,40 @@ function wp_get_webp_info( $filename ) {
  *                     that the `loading` attribute should be skipped.
  */
 function wp_get_loading_attr_default( $context ) {
-	// Only elements with 'the_content' or 'the_post_thumbnail' context have special handling.
-	if ( 'the_content' !== $context && 'the_post_thumbnail' !== $context ) {
-		return 'lazy';
-	}
-
-	// Only elements within the main query loop have special handling.
-	if ( is_admin() || ! in_the_loop() || ! is_main_query() ) {
-		return 'lazy';
-	}
-
-	// Increase the counter since this is a main query content element.
-	$content_media_count = wp_increase_content_media_count();
-
-	// If the count so far is below the threshold, return `false` so that the `loading` attribute is omitted.
-	if ( $content_media_count <= wp_omit_loading_attr_threshold() ) {
+	// Skip lazy-loading for the overall block template, as it is handled more granularly.
+	if ( 'template' === $context ) {
 		return false;
 	}
 
-	// For elements after the threshold, lazy-load them as usual.
+	// Do not lazy-load images in the header block template part, as they are likely above the fold.
+	$header_area = WP_TEMPLATE_PART_AREA_HEADER;
+	if ( "template_part_{$header_area}" === $context ) {
+		return false;
+	}
+
+	/*
+	 * The first elements in 'the_content' or 'the_post_thumbnail' should not be lazy-loaded,
+	 * as they are likely above the fold.
+	 */
+	if ( 'the_content' === $context || 'the_post_thumbnail' === $context ) {
+		// Only elements within the main query loop have special handling.
+		if ( is_admin() || ! in_the_loop() || ! is_main_query() ) {
+			return 'lazy';
+		}
+
+		// Increase the counter since this is a main query content element.
+		$content_media_count = wp_increase_content_media_count();
+
+		// If the count so far is below the threshold, return `false` so that the `loading` attribute is omitted.
+		if ( $content_media_count <= wp_omit_loading_attr_threshold() ) {
+			return false;
+		}
+
+		// For elements after the threshold, lazy-load them as usual.
+		return 'lazy';
+	}
+
+	// Lazy-load by default for any unknown context.
 	return 'lazy';
 }