This also begs the question, what do we do about portrait videos? To keep things simple, how about this:

-For a landscape video in windowed mode, rotate from portrait to landscape puts the video in fullscreen in landscape orientation. 
-For a square video in windowed mode, rotate from portrait to landscape puts the video in fullscreen. 
-For a portrait video in windowed mode, rotate from portrait to landscape does nothing.
-For a video of any type in windowed mode, rotate from landscape to portrait does nothing.
-For a video of any type in fullscreen mode, rotate from landscape to portrait puts the video in windowed mode.
-For a video of any type in fullscreen mode, rotate from portrait to landscape does nothing.

Does that make sense?

Just keeping in mind the cases when we trigger the feature, conclusion from what I read from you is:
- landscape and square videos should behave the same
- portrait video shouldn't work

Is that a fair summary?

The current code (currently in review) does this:
- landscape video goes fullscreen when rotating from portrait to landscape;
- portrait video goes fullscreen when rotating from landscape to portrait;
- square videos do nothing.

Given that we are targeting phones, I would be fine with ignoring portrait videos so the feature can be easily understood by users: rotating from portrait to landscape will trigger fullscreen on non-portrait videos.

Yes, that is a good summary. We can always add portrait functionality later.

The code currently implements "enter/exit fullscreen if device is rotated to/from the orientation of the video" (so rotating from landscape to portrait would make a portrait video go fullscreen).

I suspect that's more intuitive than disabling the feature for portrait videos. I'll leave it like this for now so you guys can see how it feels, and after that we can easily disable it for portrait videos if necessary.

(Though according to Media.VideoVisibleAspectRatio, only 4% of videos are portrait anyway)

I'll treat square videos as if they were landscape for now, as suggested in #2 (though #0's idea of treating them as both landscape and portrait might be an interesting refinement for later).

Over to dahlke@ to decide if we're happy with the current approach (enter/exit fullscreen if device is rotated to/from the orientation of the video, with square videos treated as landscape). If so, this can be closed :)

I pinged dahlke@ and he said we can close this if there were no other objections.