wp-includes/block-editor.php:867Initialize site preview.
function wp_initialize_site_preview_hooks() { if ( ! defined( 'IFRAME_REQUEST' ) && isset( $_GET['wp_site_preview'] ) && 1 === (int) $_GET['wp_site_preview'] && current_user_can( 'edit_theme_options' ) ) { define( 'IFRAME_REQUEST', true ); }}Introduced in 6.8.0. Unchanged from 6.8.8 through 7.1.0.
Signature, return type and hooks compared across 4 parsed releases.
src/wp-includes/block-editor.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.