diff --git a/wp-admin/edit-form-blocks.php b/wp-admin/edit-form-blocks.php
index 0f4ccee94b..617432d957 100644
--- a/wp-admin/edit-form-blocks.php
+++ b/wp-admin/edit-form-blocks.php
@@ -202,6 +202,29 @@ if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) {
 	}
 }
 
+// Image sizes.
+$image_sizes   = get_intermediate_image_sizes();
+$image_sizes[] = 'full';
+
+/** This filter is documented in wp-admin/includes/media.php */
+$image_size_names = apply_filters(
+	'image_size_names_choose',
+	array(
+		'thumbnail' => __( 'Thumbnail' ),
+		'medium'    => __( 'Medium' ),
+		'large'     => __( 'Large' ),
+		'full'      => __( 'Full Size' ),
+	)
+);
+
+$available_image_sizes = array();
+foreach ( $image_sizes as $image_size_slug ) {
+	$available_image_sizes[] = array(
+		'slug' => $image_size_slug,
+		'name' => isset( $image_size_names[ $image_size_slug ] ) ? $image_size_names[ $image_size_slug ] : $image_size_slug,
+	);
+}
+
 // Lock settings.
 $user_id = wp_check_post_lock( $post->ID );
 if ( $user_id ) {
@@ -257,12 +280,17 @@ $editor_settings = array(
 	'maxUploadFileSize'      => $max_upload_size,
 	'allowedMimeTypes'       => get_allowed_mime_types(),
 	'styles'                 => $styles,
+	'availableImageSizes'    => $available_image_sizes,
 	'postLock'               => $lock_details,
 	'postLockUtils'          => array(
 		'nonce'       => wp_create_nonce( 'lock-post_' . $post->ID ),
 		'unlockNonce' => wp_create_nonce( 'update-post_' . $post->ID ),
 		'ajaxUrl'     => admin_url( 'admin-ajax.php' ),
 	),
+
+	// Whether or not to load the 'postcustom' meta box is stored as a user meta
+	// field so that we're not always loading its assets.
+	'enableCustomFields'     => (bool) get_user_meta( get_current_user_id(), 'enable_custom_fields', true ),
 );
 
 $autosave = wp_get_post_autosave( $post_ID );
diff --git a/wp-admin/includes/meta-boxes.php b/wp-admin/includes/meta-boxes.php
index 0f175892d6..3ae21af404 100644
--- a/wp-admin/includes/meta-boxes.php
+++ b/wp-admin/includes/meta-boxes.php
@@ -1463,11 +1463,21 @@ function register_and_do_post_meta_boxes( $post ) {
 	}
 
 	if ( post_type_supports( $post_type, 'custom-fields' ) ) {
-		$args = array(
-			'__back_compat_meta_box'             => ! (bool) get_user_meta( get_current_user_id(), 'enable_custom_fields', true ),
-			'__block_editor_compatible_meta_box' => true,
-		);
-		add_meta_box( 'postcustom', __( 'Custom Fields' ), 'post_custom_meta_box', null, 'normal', 'core', $args );
+		$screen = get_current_screen();
+		if ( ! $screen || ! $screen->is_block_editor() || (bool) get_user_meta( get_current_user_id(), 'enable_custom_fields', true ) ) {
+			add_meta_box(
+				'postcustom',
+				__( 'Custom Fields' ),
+				'post_custom_meta_box',
+				null,
+				'normal',
+				'core',
+				array(
+					'__back_compat_meta_box'             => false,
+					'__block_editor_compatible_meta_box' => true,
+				)
+			);
+		}
 	}
 
 	/**
diff --git a/wp-admin/includes/post.php b/wp-admin/includes/post.php
index 7b07e1dc3f..dfe85ac1cd 100644
--- a/wp-admin/includes/post.php
+++ b/wp-admin/includes/post.php
@@ -2112,7 +2112,7 @@ function get_block_categories( $post ) {
 		array(
 			'slug'  => 'common',
 			'title' => __( 'Common Blocks' ),
-			'icon'  => 'screenoptions',
+			'icon'  => null,
 		),
 		array(
 			'slug'  => 'formatting',
@@ -2214,6 +2214,10 @@ function the_block_editor_meta_boxes() {
 	<form class="metabox-base-form">
 	<?php the_block_editor_meta_box_post_form_hidden_fields( $post ); ?>
 	</form>
+	<form id="toggle-custom-fields-form" method="post" action="<?php echo esc_attr( admin_url( 'post.php' ) ); ?>">
+		<?php wp_nonce_field( 'toggle-custom-fields' ); ?>
+		<input type="hidden" name="action" value="toggle-custom-fields" />
+	</form>
 	<?php foreach ( $locations as $location ) : ?>
 		<form class="metabox-location-<?php echo esc_attr( $location ); ?>" onsubmit="return false;">
 			<div id="poststuff" class="sidebar-open">
@@ -2289,6 +2293,29 @@ function the_block_editor_meta_boxes() {
 		printf( "<script type='text/javascript'>\n%s\n</script>\n", trim( $script ) );
 	}
 
+	/**
+	 * If the 'postcustom' meta box is enabled, then we need to perform some
+	 * extra initialization on it.
+	 */
+	$enable_custom_fields = (bool) get_user_meta( get_current_user_id(), 'enable_custom_fields', true );
+	if ( $enable_custom_fields ) {
+		$script = "( function( $ ) {
+			if ( $('#postcustom').length ) {
+				$( '#the-list' ).wpList( {
+					addBefore: function( s ) {
+						s.data += '&post_id=$post->ID';
+						return s;
+					},
+					addAfter: function() {
+						$('table#list-table').show();
+					}
+				});
+			}
+		} )( jQuery );";
+		wp_enqueue_script( 'wp-lists' );
+		wp_add_inline_script( 'wp-lists', $script );
+	}
+
 	// Reset meta box data.
 	$wp_meta_boxes = $_original_meta_boxes;
 }
diff --git a/wp-admin/post.php b/wp-admin/post.php
index 3a899cb4f3..80d9086db8 100644
--- a/wp-admin/post.php
+++ b/wp-admin/post.php
@@ -312,6 +312,18 @@ switch ( $action ) {
 		wp_redirect( $url );
 		exit();
 
+	case 'toggle-custom-fields':
+		check_admin_referer( 'toggle-custom-fields' );
+
+		$current_user_id = get_current_user_id();
+		if ( $current_user_id ) {
+			$enable_custom_fields = (bool) get_user_meta( $current_user_id, 'enable_custom_fields', true );
+			update_user_meta( $current_user_id, 'enable_custom_fields', ! $enable_custom_fields );
+		}
+
+		wp_safe_redirect( wp_get_referer() );
+		exit();
+
 	default:
 		/**
 		 * Fires for a given custom post action request.
diff --git a/wp-includes/script-loader.php b/wp-includes/script-loader.php
index d17b769773..82ea66d700 100644
--- a/wp-includes/script-loader.php
+++ b/wp-includes/script-loader.php
@@ -235,6 +235,7 @@ function wp_default_packages_scripts( &$scripts ) {
 		'data'                               => array(
 			'lodash',
 			'wp-compose',
+			'wp-deprecated',
 			'wp-element',
 			'wp-is-shallow-equal',
 			'wp-polyfill',
@@ -351,6 +352,7 @@ function wp_default_packages_scripts( &$scripts ) {
 			'lodash',
 			'wp-blocks',
 			'wp-data',
+			'wp-deprecated',
 			'wp-escape-html',
 			'wp-polyfill',
 		),
diff --git a/wp-includes/version.php b/wp-includes/version.php
index a850ec8331..9d4eccdadb 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44257';
+$wp_version = '5.1-alpha-44260';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.