Index: src/wp-admin/edit-form-blocks.php
===================================================================
--- src/wp-admin/edit-form-blocks.php	(revision 44724)
+++ src/wp-admin/edit-form-blocks.php	(working copy)
@@ -23,9 +23,7 @@
  */
 global $post_type, $post_type_object, $post, $title, $editor_styles, $wp_meta_boxes;
 
-// Flag that we're loading the block editor.
 $current_screen = get_current_screen();
-$current_screen->is_block_editor( true );
 
 /*
  * Emoji replacement is disabled for now, until it plays nicely with React.
Index: src/wp-admin/includes/class-wp-screen.php
===================================================================
--- src/wp-admin/includes/class-wp-screen.php	(revision 44724)
+++ src/wp-admin/includes/class-wp-screen.php	(working copy)
@@ -202,9 +202,10 @@
 			return $hook_name;
 		}
 
-		$post_type = $taxonomy = null;
-		$in_admin  = false;
-		$action    = '';
+		$post_type       = $taxonomy = null;
+		$in_admin        = false;
+		$action          = '';
+		$is_block_editor = false;
 
 		if ( $hook_name ) {
 			$id = $hook_name;
@@ -293,7 +294,8 @@
 					if ( $post_id ) {
 						$post = get_post( $post_id );
 						if ( $post ) {
-							$post_type = $post->post_type;
+							$post_type       = $post->post_type;
+							$is_block_editor = use_block_editor_for_post( $post );
 						}
 					}
 					break;
@@ -357,13 +359,14 @@
 			$screen->id = $id;
 		}
 
-		$screen->base       = $base;
-		$screen->action     = $action;
-		$screen->post_type  = (string) $post_type;
-		$screen->taxonomy   = (string) $taxonomy;
-		$screen->is_user    = ( 'user' == $in_admin );
-		$screen->is_network = ( 'network' == $in_admin );
-		$screen->in_admin   = $in_admin;
+		$screen->base            = $base;
+		$screen->action          = $action;
+		$screen->post_type       = (string) $post_type;
+		$screen->taxonomy        = (string) $taxonomy;
+		$screen->is_user         = ( 'user' == $in_admin );
+		$screen->is_network      = ( 'network' == $in_admin );
+		$screen->in_admin        = $in_admin;
+		$screen->is_block_editor = $is_block_editor;
 
 		self::$_registry[ $id ] = $screen;