Silent failure loading page application in iframe over https

Problem

I have an application driving a tab on a client's page. The application works correctly if the user has not enabled FB's "secure browsing" feature. If attempting to开发者_StackOverflow view over HTTPS, the iframe doesn't even appear (no errors, no mixed-content warnings). When correctly loading over HTTP, the div with the id "pagelet_app_runner" has an iframe inserted into it and the application content is loaded inside there. Over HTTPS, this div remains empty and the iframe is not inserted into the page. There are no Javascript errors appearing in Firebug or Chrome's equivalent console.

Why I'm Asking Here

The host has a valid SSL certificate and there is no 'mixed content' at the URL in question. I can successfully view the content over HTTP or HTTPS by visiting the URL directly, and I can do the same by visiting apps.facebook.com/canvasURL/tabURL. It is only when attempting to view within a Page Tab that the HTTPS load fails as described above. My application is configured with both regular and secure canvas and tab URLs.

Attempted Debugging

I've recorded some sessions with Charles but since the iframe isn't being inserted into the page, I think I'm coming at the problem after it's already occured. I'm no Charles expert so happy to be corrected here.

Apache isn't seeing any request (in either regular or ssl logs) for the affected loads. non-SSL loads come through as expected in access_log.

Plea for Help

I'm out of ideas for debugging this. Does anybody have any suggestions? What really obvious and stupid mistake might I have made? :)

edit: nicer formatting

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Your app canvas URL is https://skinnycomp.nextstudio.com.au/skinnycowcomps/ , which send 404 error to Facebook proxy (request is going through proxy when viewing app via tab), also when viewing your app via apps (https://apps.facebook.com/122381834451561/), again 404... maybe Facebook proxy is ignoring 404 and posting blank... Try changing canvas URL to https://skinnycomp.nextstudio.com.au/skinnycowcomps/tab, also you can check if your app is accessed via page tab, in signed_request there should be page_id...

23:51:15.379[549ms][total 1667ms] Status: 404[Not Found]
GET https://skinnycomp.nextstudio.com.au/skinnycowcomps/

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This is a real longshot since I'm sure you've triple checked all the settings, but the blank page can happen if an invalid url is specified in the Page Tab URL field in the app settings. Since it only happens on https, it would imply something specifically with the Secure Page Tab URL entry. It might be worth checking that again, and maybe even re-saving it or changing it to something else to see if it helps.

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I was using relative URLs for the regular and secure tab URL fields. From memory relative URLs here were mandatory at some point in the past. It appears now that a relative URL will still work for HTTP but not for HTTPs. Fix: absolute URLs. Hopefully FB update their field validation to match what's required too.