Media: Prevent special images within post content to skew image counts and cause lazy-loading bugs.

In order to skip lazy-loading the first few images on a page, as of WordPress 5.9 there has been logic to count images that are eligible based on certain criteria. One of those groups are images that appear within the content of a post.

This changeset fixes a bug where images created via `get_the_post_thumbnail()` or `wp_get_attachment_image()` that are injected into the post content would skew the count and therefore result in all images to be lazy-loaded, potentially hurting load time performance. This is relevant for example when those functions are called in server-side rendered blocks, or any other filter callbacks hooked into `the_content`.

Props flixos90, antpb, joedolson, spacedmonkey, mukesh27, thekt12, costdev, jrf.
Fixes #58089.
See #53675.


git-svn-id: https://develop.svn.wordpress.org/trunk@55825 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -5506,6 +5506,16 @@ function wp_get_loading_attr_default( $context ) {
 		return false;
 	}
 
+	/*
+	 * Skip programmatically created images within post content as they need to be handled together with the other
+	 * images within the post content.
+	 * Without this clause, they would already be counted below which skews the number and can result in the first
+	 * post content image being lazy-loaded only because there are images elsewhere in the post content.
+	 */
+	if ( ( 'the_post_thumbnail' === $context || 'wp_get_attachment_image' === $context ) && doing_filter( 'the_content' ) ) {
+		return false;
+	}
+
 	/*
 	 * The first elements in 'the_content' or 'the_post_thumbnail' should not be lazy-loaded,
 	 * as they are likely above the fold.