From what the Fibery team have said before, they view Back button as a standard and critical part of navigation in Fibery. I personally do too, and in many other apps. To the point where I find the poor handling of it in Notion to be quite an annoyance. However it’s true that not every mouse has a “back” button, and not every user knows about touchpad back gestures. So there is something to be said for making these things a bit clearer and more consistent.
I would say really that regardless of whether it is confusing or a navigation challenge to go between pop-up and full-view, it is arguably an inconsistency or seemingly arbitrary choice, which perhaps should be harmonized/clarified. That said I don’t think the full view should have an “x” because it’s not “over” anything, theoretically. If they are to provide any control for it I think there should always be a way to go from full view to pop-up view. If there was no contents “behind” that should now be shown, displaying a white page is fine. Hey, maybe someone is testing the pop-up view to see how something looks in it while they edit, or whatever. The point is it should be consistent, you shouldn’t be able to go to full view without a button to also get you back to pop-up…