Script Loader: Move delayed head script to footer when there is a blocking footer dependent.

This prevents a performance regression when a blocking script is enqueued in the footer which depends on a delayed script in the `head` (with `async` or `defer`). In order to preserve the execution order, a delayed dependency must fall back to blocking when there is a blocking dependent. But since it was originally delayed (and thus executes similarly to a footer script), it does not need to be in the head and can be moved to the footer. This prevents blocking the critical rendering path.

Props adamsilverstein, westonruter, flixos90.
Fixes #59599.
See #12009.

Built from https://develop.svn.wordpress.org/trunk@56933


git-svn-id: http://core.svn.wordpress.org/trunk@56444 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-scripts.php

@@ -231,6 +231,25 @@ class WP_Scripts extends WP_Dependencies {
 		return true;
 	}
 
+	/**
+	 * Checks whether all dependents of a given handle are in the footer.
+	 *
+	 * If there are no dependents, this is considered the same as if all dependents were in the footer.
+	 *
+	 * @since 6.4.0
+	 *
+	 * @param string $handle Script handle.
+	 * @return bool Whether all dependents are in the footer.
+	 */
+	private function are_all_dependents_in_footer( $handle ) {
+		foreach ( $this->get_dependents( $handle ) as $dep ) {
+			if ( isset( $this->groups[ $dep ] ) && 0 === $this->groups[ $dep ] ) {
+				return false;
+			}
+		}
+		return true;
+	}
+
 	/**
 	 * Processes a script dependency.
 	 *
@@ -281,6 +300,25 @@ class WP_Scripts extends WP_Dependencies {
 			$intended_strategy = '';
 		}
 
+		/*
+		 * Move this script to the footer if:
+		 * 1. The script is in the header group.
+		 * 2. The current output is the header.
+		 * 3. The intended strategy is delayed.
+		 * 4. The actual strategy is not delayed.
+		 * 5. All dependent scripts are in the footer.
+		 */
+		if (
+			0 === $group &&
+			0 === $this->groups[ $handle ] &&
+			$intended_strategy &&
+			! $this->is_delayed_strategy( $strategy ) &&
+			$this->are_all_dependents_in_footer( $handle )
+		) {
+			$this->in_footer[] = $handle;
+			return false;
+		}
+
 		if ( $conditional ) {
 			$cond_before = "<!--[if {$conditional}]>\n";
 			$cond_after  = "<![endif]-->\n";

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-beta4-56932';
+$wp_version = '6.4-beta4-56933';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.