Add metabox flag for RTC-compatible metaboxes

src/wp-admin/includes/post.php

@@ -2429,10 +2429,17 @@ function the_block_editor_meta_boxes() {
 					continue;
 				}
 
-				$meta_boxes_per_location[ $location ][] = array(
+				$meta_box_entry = array(
 					'id'    => $meta_box['id'],
 					'title' => $meta_box['title'],
 				);
+
+				// Mark meta boxes compatible with real-time collaboration.
+				if ( ! empty( $meta_box['args']['__rtc_compatible_meta_box'] ) ) {
+					$meta_box_entry['__rtc_compatible'] = true;
+				}
+
+				$meta_boxes_per_location[ $location ][] = $meta_box_entry;
 			}
 		}
 	}