I’ve taken a look at this and regretfully it isn’t something that we’ll be able to support or make possible in the builder. Allow me to elaborate a bit on what’s going on here.
- The Pro builders work by having the main editing frame, and an iframe to manage the live preview
- Data is constantly being synchronized between the two frames
- Due to browser security rules, the only time the two frames can actually communicate is if they are on the same domain.
- Because of this limitation, we detect if the URLs match and show the error message.
On a technical level, browsers offer a way around this by enabling Cross Origin Resource Sharing but that would involve a special setup in the server configuration. It also means we’d need to retool how our HTTP requests work to offer cors and standard setups. Regretfully we’ve opted not to pursue this due to how involved it is from a development standpoint and also how it introduces potential security issues if used improperly.
You can still edit elements in the outline and save the page. So it’s possible to modify the page without a live preview. It’d be cumbersome, but if you saved often and refreshed another tab/window with the front end open you could see the changes.
Alternatively, you could look into ways to allow the domain mapping to take place, but also allow the page to be accessible at the original URL on the primary domain. I’m not sure if this is possible, but if that was an option you could perhaps give it a more obscure URL and noindex it.
I’m sorry it isn’t the answer you may have been looking for, but I hope this offers some clarity around what’s going on here.
Unfortunately we can’t do much more than offer the few workarounds described here.