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The iFrame in this pic is a full featured app with complex structure. The app is embedded into a popular social networking site called vk.com. iFrame apps on this site work similar to Facebook's "custom tabs". So basically it's a site inside another site.
How to implement "Go back" navigation in order to not confuse users with secondary breadcrumbs or back button in this case?
I hope and would suggest, your app's color scheme isn't the same as parent site in this case vk.com. because that will be the last thing you want, as it would really confuse the end user.
So you can try below options for placing your back buttons or links:
It is a matter of indicating clearly where is the limit. You can achieve this with a clear border if the background-color (and lightness) of the inside and outside is similar:
Or you can achieve it giving a very different background-color without the need of the previous border:
