We are using WordPress MU Domain Mapping with path-based sites. So the main site is http://DomainA.com (with no path and no mapping) and the secondary site is http://DomainA.com/SiteB internally, but mapped to http://DomainB.com. On the DomainA.com homepage, there is a sidebar that displays a link to the latest post from DomainB. The link normally looks something like http://DomainB.com/the-post-slug.
So far, so good. This is all working great. However, we recently received a complaint from a user who somehow ended up at https://DomainA.com. Not sure quite how that happened, but it is an ecommerce site (using WooCommerce), so maybe HTTPS lingered after checkout. That doesn't really matter, it's not a problem by itself. Everything on the DomainA.com homepage works fine in HTTPS, except that the link to the latest post from DomainB loses the domain mapping, so it becomes https://DomainA.com/SiteB/the-post-slug.
That's not ideal, but wouldn't be a huge problem if the link at least worked. Unfortunately, when following that link the server automatically redirects to the domain mapped DomainB.com, which would be the preferred behavior EXCEPT for the fact that it doesn't correct the protocol from HTTPS to HTTP. So the user ends up at https://DomainB.com/the-post-slug. We don't have and don't need an SSL certificate for DomainB.com (that site is blog-only, no ecommerce), so this results in a security warning.
The ideal solution here would be to have all links from DomainA to DomainB ALWAYS point to http://DomainB.com/*, even when accessing DomainA via HTTPS. So far, though, I haven't been able to find any way to control that at all. If that's not possible I'm certainly open to alternate workarounds.
Any suggestions here would be appreciated. Thanks.