Some videos (basically any video on Disney+) will have hardcoded black bars on the top and bottom of the image (letterboxing) to make the video better fit most physical screen dimensions. This is however not a relevant concern when viewing the video in PIP, and it would be nice with a solution to crop these black bars out
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This is unfortunately not something that would be trivial for us to implement, so I'd say even though it would indeed represent an improvement to the experience, it's not on our next 3-6 months' backlog.