Media: Skip cross-origin isolation for third-party page builders.

Add a check to skip Document-Isolation-Policy when a third-party page
builder overrides the block editor via a custom action parameter. DIP
isolates the document into its own agent cluster, which blocks
same-origin iframe access that these editors rely on.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: adamsilverstein

src/wp-includes/media.php

@@ -6430,6 +6430,14 @@ function wp_set_up_cross_origin_isolation(): void {
 		return;
 	}
 
+	// Skip when a third-party page builder overrides the block editor.
+	// DIP isolates the document into its own agent cluster,
+	// which blocks same-origin iframe access that these editors rely on.
+	// phpcs:ignore WordPress.Security.NonceVerification.Recommended
+	if ( isset( $_GET['action'] ) && 'edit' !== $_GET['action'] ) {
+		return;
+	}
+
 	// Cross-origin isolation is not needed if users can't upload files anyway.
 	if ( ! current_user_can( 'upload_files' ) ) {
 		return;