Comment 4 for bug 1767887

This looks to be due to the change in htdocs/theme/raw/sass/form/_site-messages.scss

        + .main-content {
            @media (min-width: $screen-sm-min) {
                margin-top: $navbar-height + ($i * 33px);
            }
        }

As it produces something like (when there are 2 site messages):

@media (min-width:768px) {
  .header.message-count-2+.main-content{margin-top:126px}
}

And the positioning of the selector dropdown is 126px out of alignment