Here are some updated tests I ran using the Xcode iPhone X simulator. I used the full screen options to play back a 21:9 video sample.
Using Safari I played this 21:9 video off Youtube.com. Please note this is not using the YouTube app only the safari browser. youtu.be/iGnwVyQFT4A. This is what you can expect of any videos with a wider aspect ratio than 16:9. You can expect similar results from 2.35:1 or 2.40:1 aspect ratios. Here is a link to many examples of movies wider than 16:9 http://www.imdb.com/list/ls073624685/
As you can see I have examples of the video playing uncropped (avoiding the notch) and zoomed in where the notch and rounded corners will impact some of the image on screen. Note: the video was playing and the captured frames don't match exactly on each device. The 21:9 video does not appear to be trimmed on the top bottom or sides of the iPhone X when zoomed in and would seem to fit the screen exactly (with the exception of the corner and notch cut outs.)
I also added an example playing the same video using the iPhone X's build in media player. This image was taken with the video zoomed out to avoid the notch. It appears to be a little smaller than the same video playing back on YouTube. This is probably a software issue and will be fixed.
Finally in the last image I overlaid the iPhone X on top of an iPhone 8 Plus with the video uncropped. The slightly larger video is coming from the iPhone X. I set the image to about 70% opacity so the iPhone 8 Plus's (smaller) video screen can be seen below the X. When playing 21:9 videos on an 8 Plus the image is smaller (if only by a few pixels)
Adding for Roeiz
Using Safari I played this 21:9 video off Youtube.com. Please note this is not using the YouTube app only the safari browser. youtu.be/iGnwVyQFT4A. This is what you can expect of any videos with a wider aspect ratio than 16:9. You can expect similar results from 2.35:1 or 2.40:1 aspect ratios. Here is a link to many examples of movies wider than 16:9 http://www.imdb.com/list/ls073624685/
As you can see I have examples of the video playing uncropped (avoiding the notch) and zoomed in where the notch and rounded corners will impact some of the image on screen. Note: the video was playing and the captured frames don't match exactly on each device. The 21:9 video does not appear to be trimmed on the top bottom or sides of the iPhone X when zoomed in and would seem to fit the screen exactly (with the exception of the corner and notch cut outs.)
I also added an example playing the same video using the iPhone X's build in media player. This image was taken with the video zoomed out to avoid the notch. It appears to be a little smaller than the same video playing back on YouTube. This is probably a software issue and will be fixed.
Finally in the last image I overlaid the iPhone X on top of an iPhone 8 Plus with the video uncropped. The slightly larger video is coming from the iPhone X. I set the image to about 70% opacity so the iPhone 8 Plus's (smaller) video screen can be seen below the X. When playing 21:9 videos on an 8 Plus the image is smaller (if only by a few pixels)
Adding for Roeiz
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