I've set up my FB app so that it has a Mobile Web URL component. I've noted from testing that whenever I navigate to canvas URL of my app on a mobile device, Facebook automatically redirects the mobile to the defined Mobile Web URL.
If I do the same from desktop, the canvas URL is loaded in to the browser. In other words, if FB detects a mobile device when navigating to an app's canvas URL, it will immediately redirect to a defined Mobile Web URL and not load the canvas URL content.
The problem with this is that when the canvas URL loads, FB passes to it a signed_request form parameter. Yet, when it redirects to the Mobile Web URL it does not.
I noticed when checking a network sniffer that when on mobile Facebook redirects the app.facebook.com URL to m.facebook.com, and thought perhaps the signed_request was getting lost along the way. But when checking the first call to the canvas URL there was no signed_request there either.
It's like Facebook only provides the signed_request when it processes the canvas URL, which in the case of mobile is never.
Has anyone any insight or advice on how to get the signed_request? My only thought at the moment is forcing the user to authorise the app on mobile as soon as they are redirected, but i'd rather not do that if possible (I'm not a fan of gating techniques)
--- EDIT ---
Closing - see my answer.