A workaround that allows you to use the MER Cart widget within a sidebar on the registration-checkout page. This function simply removes the #checkout bookmark from the link to prevent X Theme from simply scrolling to that bookmark.

functions.php

<?php //Please do not include the opening php tag if you already have one.

//This function simply removes the '#checkout' book mark from the view event cart link used within the MER Cart Widget.
//This is a workaround for using the Widget on the registration-checkout with theme X.
function tw_ee_remove_checkout_hash_from_cart_widget( $template_args ) {

	if( array_key_exists( 'view_event_cart_url', $template_args ) ) {

		$template_args[ 'view_event_cart_url' ] = str_replace( '#checkout', '', $template_args[ 'view_event_cart_url' ] );

	}

	return $template_args;

}
add_filter( 'FHEE__EEH_Template__display_template__template_args', 'tw_ee_remove_checkout_hash_from_cart_widget' );

Ref - https://eventespresso.com/topic/view-event-cart-glitch-not-working/#post-208659