Add the current post ID to the Customize admin menu URL

something.php

/**
 * Add the current post ID to the Customize admin menu URL
 *
 * @internal Called via wp_before_admin_bar_render action
 *
 * @param null
 *
 * @return null
 */
function pmc_add_post_id_to_customizer_url(){
	global $wp_admin_bar, $post;

	$customize_node = $wp_admin_bar->get_node( 'customize' );

	if ( is_object( $customize_node ) ) {

		// Remove the existing Customizer node
		$wp_admin_bar->remove_node( 'customize' );

		// Add the Customize Menu back to the admin bar
		$wp_admin_bar->add_menu( array(
			'id'     => 'customize',
			'title'  => __( 'Customize' ),

			// Add &post_id=XXXX to the Admin Bar Customizer URL
			'href'   => add_query_arg(
				'post_id',
				$post->ID,
				$customize_node->href
			),
			'meta'   => array(
				'class' => 'hide-if-no-customize',
			),
		) );
	}
}
add_action( 'wp_before_admin_bar_render', 'pmc_add_post_id_to_customizer_url', 10, 0 );