Script Loader: Prefetch assets for the next admin screen

src/wp-admin/admin-header.php

@@ -95,6 +95,10 @@
 <title><?php echo esc_html( $admin_title ); ?></title>
 <?php
 
+/*
+ * The dependencies of `colors` and the `utils` script are prefetched from the login screen by
+ * wp_prefetch_admin_assets(), which needs updating if these change.
+ */
 wp_enqueue_style( 'colors' );
 wp_enqueue_script( 'utils' );
 wp_enqueue_script( 'svg-painter' );

src/wp-admin/admin.php

@@ -118,6 +118,10 @@
 $date_format = __( 'F j, Y' );
 $time_format = __( 'g:i a' );
 
+/*
+ * The `jquery` dependency of this script is prefetched from the login screen by
+ * wp_prefetch_admin_assets(), which needs updating if this changes.
+ */
 wp_enqueue_script( 'common' );
 
 /**

src/wp-admin/edit-form-blocks.php

@@ -342,6 +342,7 @@ static function ( $classes ) {
 /**
  * Styles
  */
+// Prefetched for the block editor by wp_prefetch_admin_assets(), which needs updating if this changes.
 wp_enqueue_style( 'wp-edit-post' );
 
 /**

src/wp-includes/class-wp-admin-bar.php

@@ -70,6 +70,7 @@ public function initialize() {
 		add_action( 'wp_head', $header_callback );
 
 		wp_enqueue_script( 'admin-bar' );
+		// Prefetched from the login screen by wp_prefetch_admin_assets(), which needs updating if this changes.
 		wp_enqueue_style( 'admin-bar' );
 
 		/**

src/wp-includes/class-wp-editor.php

@@ -913,7 +913,10 @@ public static function enqueue_default_editor() {
 
 		self::enqueue_scripts( true );
 
-		// Also add wp-includes/css/editor.css.
+		/*
+		 * Also add wp-includes/css/editor.css. It is prefetched for the block editor by
+		 * wp_prefetch_admin_assets(), which needs updating if this changes.
+		 */
 		wp_enqueue_style( 'editor-buttons' );
 
 		if ( is_admin() ) {

src/wp-includes/class-wp-scripts.php

@@ -415,39 +415,7 @@ public function do_item( $handle, $group = false ) {
 			return true;
 		}
 
-		if ( ! preg_match( '|^(https?:)?//|', $src ) && ! ( $this->content_url && str_starts_with( $src, $this->content_url ) ) ) {
-			$src = $this->base_url . $src;
-		}
-
-		$ver_to_add = '';
-		if ( empty( $obj->ver ) && null !== $obj->ver && is_string( $this->default_version ) ) {
-			$ver_to_add = $this->default_version;
-		} elseif ( is_scalar( $obj->ver ) ) {
-			$ver_to_add = (string) $obj->ver;
-		}
-
-		$added_args = (string) ( $this->args[ $handle ] ?? '' );
-
-		if ( '' !== $ver_to_add || '' !== $added_args ) {
-			$fragment = strstr( $src, '#' );
-			if ( false !== $fragment ) {
-				$src = substr( $src, 0, -strlen( $fragment ) );
-			}
-
-			if ( '' !== $ver_to_add ) {
-				$src .= ( str_contains( $src, '?' ) ? '&' : '?' ) . 'ver=' . rawurlencode( $ver_to_add );
-			}
-			if ( '' !== $added_args ) {
-				$src .= ( str_contains( $src, '?' ) ? '&' : '?' ) . $added_args;
-			}
-
-			if ( false !== $fragment ) {
-				$src .= $fragment;
-			}
-		}
-
-		/** This filter is documented in wp-includes/class-wp-scripts.php */
-		$src = esc_url_raw( apply_filters( 'script_loader_src', $src, $handle ) );
+		$src = $this->get_src( $handle );
 
 		if ( ! $src ) {
 			return true;
@@ -506,6 +474,67 @@ public function do_item( $handle, $group = false ) {
 		return true;
 	}
 
+	/**
+	 * Gets the URL a registered script is loaded from.
+	 *
+	 * This is the URL printed in the script's `src` attribute, including the version query
+	 * argument and any arguments added to the handle, after the {@see 'script_loader_src'} filter.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @param string $handle Script handle.
+	 * @return string Script URL, or an empty string when the script is not registered, has no
+	 *                source of its own because it only aliases other scripts, or was filtered away.
+	 */
+	public function get_src( string $handle ): string {
+		if ( ! isset( $this->registered[ $handle ] ) ) {
+			return '';
+		}
+
+		$obj = $this->registered[ $handle ];
+		$src = $obj->src;
+
+		if ( ! $src || ! is_string( $src ) ) {
+			return '';
+		}
+
+		if ( ! preg_match( '|^(https?:)?//|', $src ) && ! ( $this->content_url && str_starts_with( $src, $this->content_url ) ) ) {
+			$src = $this->base_url . $src;
+		}
+
+		$ver_to_add = '';
+		if ( empty( $obj->ver ) && null !== $obj->ver && is_string( $this->default_version ) ) {
+			$ver_to_add = $this->default_version;
+		} elseif ( is_scalar( $obj->ver ) ) {
+			$ver_to_add = (string) $obj->ver;
+		}
+
+		$added_args = (string) ( $this->args[ $handle ] ?? '' );
+
+		if ( '' !== $ver_to_add || '' !== $added_args ) {
+			$fragment = strstr( $src, '#' );
+			if ( false !== $fragment ) {
+				$src = substr( $src, 0, -strlen( $fragment ) );
+			}
+
+			if ( '' !== $ver_to_add ) {
+				$src .= ( str_contains( $src, '?' ) ? '&' : '?' ) . 'ver=' . rawurlencode( $ver_to_add );
+			}
+			if ( '' !== $added_args ) {
+				$src .= ( str_contains( $src, '?' ) ? '&' : '?' ) . $added_args;
+			}
+
+			if ( false !== $fragment ) {
+				$src .= $fragment;
+			}
+		}
+
+		/** This filter is documented in wp-includes/class-wp-scripts.php */
+		$src = apply_filters( 'script_loader_src', $src, $handle );
+
+		return is_string( $src ) ? esc_url_raw( $src ) : '';
+	}
+
 	/**
 	 * Adds extra code to a registered script.
 	 *

src/wp-includes/class-wp-styles.php

@@ -224,14 +224,9 @@ public function do_item( $handle, $group = false ) {
 		 */
 		$tag = apply_filters( 'style_loader_tag', $tag, $handle, $href, $media );
 
-		if ( 'rtl' === $this->text_direction && isset( $obj->extra['rtl'] ) && $obj->extra['rtl'] ) {
-			if ( is_bool( $obj->extra['rtl'] ) || 'replace' === $obj->extra['rtl'] ) {
-				$suffix   = $obj->extra['suffix'] ?? '';
-				$rtl_href = str_replace( "{$suffix}.css", "-rtl{$suffix}.css", $this->_css_href( $src, $ver, "$handle-rtl" ) );
-			} else {
-				$rtl_href = $this->_css_href( $obj->extra['rtl'], $ver, "$handle-rtl" );
-			}
+		$rtl_href = $this->get_rtl_href( $handle );
 
+		if ( null !== $rtl_href ) {
 			$rtl_tag = sprintf(
 				"<link rel='%s' id='%s-rtl-css'%s href='%s' media='%s' />\n",
 				$rel,
@@ -264,6 +259,54 @@ public function do_item( $handle, $group = false ) {
 		return true;
 	}
 
+	/**
+	 * Gets the URL of the right-to-left stylesheet for a registered style.
+	 *
+	 * On a right-to-left locale, a style registered with `rtl` data is served by a separate
+	 * stylesheet, which either replaces its left-to-right one (when the data is `'replace'`)
+	 * or loads alongside it.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @param string $handle The style's registered handle.
+	 * @return string|null URL of the right-to-left stylesheet, after the {@see 'style_loader_src'}
+	 *                     filter. Null when the text direction is not right-to-left, or the style is
+	 *                     not registered, has no source of its own, or has no right-to-left variant.
+	 */
+	public function get_rtl_href( string $handle ): ?string {
+		if ( 'rtl' !== $this->text_direction || ! isset( $this->registered[ $handle ] ) ) {
+			return null;
+		}
+
+		$obj = $this->registered[ $handle ];
+
+		if ( ! $obj->src || ! isset( $obj->extra['rtl'] ) || ! $obj->extra['rtl'] ) {
+			return null;
+		}
+
+		if ( null === $obj->ver ) {
+			$ver = '';
+		} else {
+			$ver = $obj->ver ? $obj->ver : $this->default_version;
+		}
+
+		if ( isset( $this->args[ $handle ] ) ) {
+			$ver = $ver ? $ver . '&amp;' . $this->args[ $handle ] : $this->args[ $handle ];
+		}
+
+		if ( is_bool( $obj->extra['rtl'] ) || 'replace' === $obj->extra['rtl'] ) {
+			$suffix = isset( $obj->extra['suffix'] ) && is_string( $obj->extra['suffix'] ) ? $obj->extra['suffix'] : '';
+			return str_replace( "{$suffix}.css", "-rtl{$suffix}.css", $this->_css_href( $obj->src, $ver, "$handle-rtl" ) );
+		}
+
+		// Any other value is the URL of the right-to-left stylesheet itself.
+		if ( ! is_string( $obj->extra['rtl'] ) ) {
+			return null;
+		}
+
+		return $this->_css_href( $obj->extra['rtl'], $ver, "$handle-rtl" );
+	}
+
 	/**
 	 * Adds extra CSS styles to a registered stylesheet.
 	 *

src/wp-includes/default-filters.php

@@ -396,6 +396,8 @@
 add_action( 'login_head', 'wp_resource_hints', 8 );
 add_action( 'login_head', 'wp_print_head_scripts', 9 );
 add_action( 'login_head', 'print_admin_styles', 9 );
+add_action( 'login_head', 'wp_prefetch_admin_assets' );
+add_action( 'admin_head', 'wp_prefetch_admin_assets' );
 add_action( 'login_head', 'wp_site_icon', 99 );
 add_action( 'login_footer', 'wp_print_footer_scripts', 20 );
 add_action( 'login_init', 'send_frame_options_header', 10, 0 );

src/wp-includes/functions.php

@@ -7664,6 +7664,7 @@ function wp_auth_check_load() {
 	 * @param WP_Screen $screen The current screen object.
 	 */
 	if ( apply_filters( 'wp_auth_check_load', $show, $screen ) ) {
+		// Prefetched from the login screen by wp_prefetch_admin_assets(), which needs updating if this changes.
 		wp_enqueue_style( 'wp-auth-check' );
 		wp_enqueue_script( 'wp-auth-check' );
 

src/wp-includes/media.php

@@ -5305,11 +5305,13 @@ function wp_enqueue_media( $args = array() ) {
 	wp_localize_script( 'media-views', '_wpMediaViewsL10n', $strings );
 
 	wp_enqueue_script( 'media-audiovideo' );
+	// Prefetched for the block editor by wp_prefetch_admin_assets(), which needs updating if this changes.
 	wp_enqueue_style( 'media-views' );
 	if ( is_admin() ) {
 		wp_enqueue_script( 'mce-view' );
 		wp_enqueue_script( 'image-edit' );
 	}
+	// Prefetched for the block editor by wp_prefetch_admin_assets(), which needs updating if this changes.
 	wp_enqueue_style( 'imgareaselect' );
 	wp_plupload_default_settings();