Always redirect user to referrer after log in if no other redirects are set.

login_redirect_to_referrer.php

<?php
/**
 * Always redirect user to referrer after log in if no other redirects are set.
 *
 */
function login_redirect_to_referrer( $redirect_to, $request, $user ) {
	
	// If already redirecting, do that.
	if ( ! empty( $redirect_to ) ) {
		return $redirect_to;
	}
		
	// Get the referrer if set.
	$redirect_to_referrer = wp_get_referer();
	if ( ! empty( $redirect_to_referrer ) ) {
		$redirect_to = $redirect_to_referrer;
	}

	return $redirect_to;
}
add_filter( 'login_redirect', 'login_redirect_to_referrer', 5, 3 );