Huh? I did watch the FB video on my computer. It occupied about 500x300 pixels, just like any embedded video. Why do I need to watch some home video in fullscreen?

I'm all about cinema and I very much appreciate a 2:35:1 aspect ratio and such, but the reality is that it doesn't matter for a lot of things, especially things often recorded on phones. Will I record in landscape when I'm doing a panorama of Mt. Everest? Sure. Do I care about the orientation of a squirrel eating out of my hand? No. Am I going to hop over the fence because the subject is too close to the camera for me to capture in landscape and I need to be 10 ft further back? No.

I'll go one step further in fact and say the vertical video of my niece is better served by a vertical video because

1) There is no important or interesting content on the left or right and she is close the camera wearing her mom's heels. If the video was landscape, you could not see her feet and all we'd see is more house walls and doorways.

2) It's a 1080p video and our TV is 4K. I can view the max native resolution even with black bars.

So yeah, we get to see my niece acting like an adult wearing heels in max native resolution on my TV. If I did it your way, it'd end up with upscaled video showing only from my nieces knees up, removing 80% of the context of the video, but with aesthetically pleasing walls and doorways.