wp-includes/customize/class-wp-customize-custom-css-setting.php:83Add filter to preview post value.
bool public function preview() { if ( $this->is_previewed ) { return false; } $this->is_previewed = true; add_filter( 'wp_get_custom_css', array( $this, 'filter_previewed_wp_get_custom_css' ), 9, 2 ); return true; }Introduced in 4.7.9. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/customize/class-wp-customize-custom-css-setting.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.