Index: src/wp-admin/admin-header.php
===================================================================
--- src/wp-admin/admin-header.php	(revision 43944)
+++ src/wp-admin/admin-header.php	(working copy)
@@ -175,7 +175,7 @@
 
 $admin_body_class .= ' no-customize-support no-svg';
 
-if ( $current_screen->is_block_editor() ) {
+if ( $current_screen->get_is_block_editor() ) {
 	// Default to is-fullscreen-mode to avoid jumps in the UI.
 	$admin_body_class .= ' block-editor-page is-fullscreen-mode wp-embed-responsive';
 
Index: src/wp-admin/edit-form-blocks.php
===================================================================
--- src/wp-admin/edit-form-blocks.php	(revision 43944)
+++ src/wp-admin/edit-form-blocks.php	(working copy)
@@ -29,7 +29,7 @@
 
 // Flag that we're loading the block editor.
 $current_screen = get_current_screen();
-$current_screen->is_block_editor( true );
+$current_screen->set_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 43944)
+++ src/wp-admin/includes/class-wp-screen.php	(working copy)
@@ -187,7 +187,7 @@
 	 * @since 5.0.0
 	 * @var bool
 	 */
-	public $is_block_editor = false;
+	protected $is_block_editor = false;
 
 	/**
 	 * Fetches a screen object.
@@ -408,18 +408,24 @@
 	}
 
 	/**
-	 * Sets or returns whether the block editor is loading on the current screen.
+	 * Sets whether the block editor is loading on the current screen.
 	 *
 	 * @since 5.0.0
 	 *
-	 * @param bool $set Optional. Sets whether the block editor is loading on the current screen or not.
+	 * @param bool $is_block_editor. Sets whether the block editor is loading on the current screen or not.
+	 */
+	public function set_is_block_editor( $is_block_editor ) {
+		$this->is_block_editor = (bool) $is_block_editor;
+	}
+
+	/**
+	 * Returns whether the block editor is loading on the current screen.
+	 *
+	 * @since 5.0.0
+	 *
 	 * @return bool True if the block editor is being loaded, false otherwise.
 	 */
-	public function is_block_editor( $set = null ) {
-		if ( $set !== null ) {
-			$this->is_block_editor = (bool) $set;
-		}
-
+	public function get_is_block_editor() {
 		return $this->is_block_editor;
 	}
 
Index: src/wp-admin/includes/template.php
===================================================================
--- src/wp-admin/includes/template.php	(revision 43944)
+++ src/wp-admin/includes/template.php	(working copy)
@@ -1154,7 +1154,7 @@
 					$block_compatible = true;
 					if ( is_array( $box[ 'args' ] ) ) {
 						// If a meta box is just here for back compat, don't show it in the block editor.
-						if ( $screen->is_block_editor() && isset( $box['args']['__back_compat_meta_box'] ) && $box['args']['__back_compat_meta_box'] ) {
+						if ( $screen->get_is_block_editor() && isset( $box['args']['__back_compat_meta_box'] ) && $box['args']['__back_compat_meta_box'] ) {
 							continue;
 						}
 
@@ -1164,7 +1164,7 @@
 						}
 
 						// If the meta box is declared as incompatible with the block editor, override the callback function.
-						if ( ! $block_compatible && $screen->is_block_editor() ) {
+						if ( ! $block_compatible && $screen->get_is_block_editor() ) {
 							$box['old_callback'] = $box['callback'];
 							$box['callback']     = 'do_block_editor_incompatible_meta_box';
 						}
@@ -1177,7 +1177,7 @@
 
 					$i++;
 					// get_hidden_meta_boxes() doesn't apply in the block editor.
-					$hidden_class = ( ! $screen->is_block_editor() && in_array( $box['id'], $hidden ) ) ? ' hide-if-js' : '';
+					$hidden_class = ( ! $screen->get_is_block_editor() && in_array( $box['id'], $hidden ) ) ? ' hide-if-js' : '';
 					echo '<div id="' . $box['id'] . '" class="postbox ' . postbox_classes($box['id'], $page) . $hidden_class . '" ' . '>' . "\n";
 					if ( 'dashboard_browser_nag' != $box['id'] ) {
 						$widget_title = $box[ 'title' ];
@@ -1196,7 +1196,7 @@
 					echo "<h2 class='hndle'><span>{$box['title']}</span></h2>\n";
 					echo '<div class="inside">' . "\n";
 
-					if ( WP_DEBUG && ! $block_compatible && 'edit' === $screen->parent_base && ! $screen->is_block_editor() && ! isset( $_GET['meta-box-loader'] ) ) {
+					if ( WP_DEBUG && ! $block_compatible && 'edit' === $screen->parent_base && ! $screen->get_is_block_editor() && ! isset( $_GET['meta-box-loader'] ) ) {
 						$plugin = _get_plugin_from_callback( $box['callback'] );
 						if ( $plugin ) {
 						?>
Index: src/wp-includes/script-loader.php
===================================================================
--- src/wp-includes/script-loader.php	(revision 43944)
+++ src/wp-includes/script-loader.php	(working copy)
@@ -2365,7 +2365,7 @@
 function wp_common_block_scripts_and_styles() {
 	global $current_screen;
 
-	if ( is_admin() && ( $current_screen instanceof WP_Screen ) && ! $current_screen->is_block_editor() ) {
+	if ( is_admin() && ( $current_screen instanceof WP_Screen ) && ! $current_screen->get_is_block_editor() ) {
 		return;
 	}
 
@@ -2399,7 +2399,7 @@
 function wp_enqueue_registered_block_scripts_and_styles() {
 	global $current_screen;
 
-	$is_editor = ( ( $current_screen instanceof WP_Screen ) && $current_screen->is_block_editor() );
+	$is_editor = ( ( $current_screen instanceof WP_Screen ) && $current_screen->get_is_block_editor() );
 
 	$block_registry = WP_Block_Type_Registry::get_instance();
 	foreach ( $block_registry->get_all_registered() as $block_name => $block_type ) {