Memory, as I understand it, is contextual. So it's not the doorway per se, but the fact that the context changed. This is why after "forgetting", your return to the original room helps you to remember.

As a side note, again as I understand it, context can be things other than physical location. It can be your emotional state (i.e., when you're happy you're more wired to recall things that happen when you were in the same / similar state of mind.

And yes, context of state of mind can be the chemical based (e.g., alcohol). Yup. The brain is a crazy and complex work of art.

I honestly felt this way about the full screen windows start menu. The old one kept me in context. I feel the same way about Gnome Shell doing it too though I think the fact that it blurs the background instead of disappearing it helps.

what constitutes a doorway ? does it need a door ? a frame ? what's the minimal set of cues that causes the effect ?

Any change in the psycho-spacial context you pegged the forgotten information to?

Hard to say what the minimum change might be; I'm sure it varies significantly from person to person.