Editor: Add 'wp-polyfill' to _wp_get_iframed_editor_assets() for iner…

…t attribute in Firefox.

Adds the polyfill script to the iframe to make the `inert` attribute work correctly in Firefox. This solves the problem that the site editor can operate on the block editor in Firefox even in browse mode.

Why?

The inert attribute is intended to render the element and its sub-tree elements `inert`, but Firefox doesn't support this attribute. For the `inert` attribute to work as intended in Firefox, even within an iframe, the script needs to be injected into the iframe as well as the global document.

References:
* [WordPress/gutenberg#47398 Gutenberg PR 47398].

Follow-up to [53160].

Props wildworks, mamaduka, youknowriad.
Fixes #57552.

git-svn-id: https://develop.svn.wordpress.org/trunk@55181 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-editor.php

@@ -298,7 +298,9 @@ function get_legacy_widget_block_editor_settings() {
 function _wp_get_iframed_editor_assets() {
 	global $pagenow;
 
-	$script_handles = array();
+	$script_handles = array(
+		'wp-polyfill',
+	);
 	$style_handles  = array(
 		'wp-edit-blocks',
 	);