Adding this snippet to the functions.php of your wordpress theme will redirect visitors to the page they were viewing after logging in.
if ( ( isset( $_GET['action'] ) && $_GET['action'] != 'logout' ) || ( isset( $_POST['login_location'] ) && ! empty( $_POST['login_location'] ) ) ) {
add_filter('login_redirect', 'my_login_redirect', 10, 3);
function my_login_redirect() {
$location = $_SERVER['HTTP_REFERER'];
wp_safe_redirect( $location );
exit();
}
}