I can see how that could be a problem with modal logins, but it doesn't have to be a show-stopper: when the modal opens, the site could history.pushState() to a

I can see how that could be a problem with modal logins, but it doesn't have to be a show-stopper: when the modal opens, the site could history.pushState() to a URL that would, when navigated to directly later, serve the page with the modal already open. Not that I know off-hand of any sites that actually do that, but the possibility exists.