Better WordPress is_admin()

functions.php

<?php
/**
 * Check if inside WP Admin. Also works in the Block Editor.
 *
 * With the introduction of Gutenberg, is_admin() was broken.
 * This better version will account for the Block Editor (Gutenberg)
 */
function mytheme_better_is_admin() {
	// Check if in Block Editor - see: https://github.com/WordPress/gutenberg/issues/51090#issuecomment-1576570247
	if ( is_admin() || ( defined( 'REST_REQUEST' ) && REST_REQUEST && 'edit' === $_GET['context'] ) ) {
		return true;
	}
	else {
		return false;
	}
}