Editor: Preload initial Site Editor canvas data

src/wp-admin/site-editor.php

@@ -237,6 +237,103 @@ static function ( $classes ) {
 	),
 );
 
+$template_slugs = array();
+$front_page     = null;
+if ( $block_editor_context->post && 'page' === $block_editor_context->post->post_type ) {
+	$template_slugs[] = empty( $block_editor_context->post->post_name ) ? 'page' : 'page-' . $block_editor_context->post->post_name;
+	$template_slugs[] = 'page';
+} else {
+	$template_slugs[] = 'front-page';
+
+	if ( 'page' === get_option( 'show_on_front' ) ) {
+		$front_page = get_post( (int) get_option( 'page_on_front' ) );
+		if ( $front_page instanceof WP_Post ) {
+			$template_slugs[] = empty( $front_page->post_name ) ? 'page' : 'page-' . $front_page->post_name;
+			$template_slugs[] = 'page';
+		}
+	} else {
+		$template_slugs[] = 'home';
+	}
+
+	$template_slugs[] = 'index';
+}
+
+$template_slugs = array_values( array_unique( $template_slugs ) );
+$templates      = get_block_templates(
+	array(
+		'slug__in' => $template_slugs,
+	),
+	'wp_template'
+);
+$priorities     = array_flip( $template_slugs );
+
+usort(
+	$templates,
+	static function ( $template_a, $template_b ) use ( $priorities ) {
+		return ( $priorities[ $template_a->slug ] ?? 999 ) - ( $priorities[ $template_b->slug ] ?? 999 );
+	}
+);
+
+$template_part_ids = array();
+$has_query         = false;
+$walk_blocks       = static function ( $blocks ) use ( &$walk_blocks, &$template_part_ids, &$has_query ) {
+	foreach ( $blocks as $block ) {
+		if ( 'core/template-part' === ( $block['blockName'] ?? '' ) && ! empty( $block['attrs']['slug'] ) ) {
+			$theme               = ! empty( $block['attrs']['theme'] ) ? $block['attrs']['theme'] : get_stylesheet();
+			$template_part_ids[] = $theme . '//' . $block['attrs']['slug'];
+		}
+
+		if ( 'core/query' === ( $block['blockName'] ?? '' ) ) {
+			$has_query = true;
+		}
+
+		if ( ! empty( $block['innerBlocks'] ) ) {
+			$walk_blocks( $block['innerBlocks'] );
+		}
+	}
+};
+
+if ( ! empty( $templates ) && ! empty( $templates[0]->content ) ) {
+	$blocks = resolve_pattern_blocks( parse_blocks( $templates[0]->content ) );
+	$walk_blocks( $blocks );
+}
+
+foreach ( array_unique( $template_part_ids ) as $template_part_id ) {
+	$preload_paths[] = '/wp/v2/template-parts/' . $template_part_id . '?context=edit';
+}
+
+if ( $front_page instanceof WP_Post ) {
+	$route_for_front_page = rest_get_route_for_post( $front_page );
+	if ( $route_for_front_page ) {
+		$preload_paths[] = add_query_arg( 'context', 'edit', $route_for_front_page );
+	}
+	$preload_paths[] = add_query_arg(
+		'slug',
+		empty( $front_page->post_name ) ? 'page' : 'page-' . $front_page->post_name,
+		'/wp/v2/templates/lookup'
+	);
+	$preload_paths[] = '/wp/v2/types/page?context=edit';
+}
+
+if ( $has_query ) {
+	$post_rest_route = rest_get_route_for_post_type_items( 'post' );
+	foreach ( array( 10, 3 ) as $per_page ) {
+		$preload_paths[] = add_query_arg(
+			array(
+				'context'       => 'edit',
+				'offset'        => 0,
+				'order'         => 'desc',
+				'orderby'       => 'date',
+				'per_page'      => $per_page,
+				'ignore_sticky' => 'false',
+			),
+			$post_rest_route
+		);
+	}
+}
+
+$preload_paths[] = '/wp/v2/taxonomies?context=view';
+
 if ( $block_editor_context->post ) {
 	$route_for_post = rest_get_route_for_post( $block_editor_context->post );
 	if ( $route_for_post ) {