Or ability to play a 21:9 movie natively on a 21:9 monitor, not cropped to the 16:9 area for all the pillarboxing?
For their massive budgets Netflix _amaze me_ with their lack of UX clout and ability.
It should be the simplest bloody thing for someone with their resources to offer keyboard shortcuts yet they've fouled the UX up in a major way. The main thing I've got against Netflix is that you have to full size the screen to hide the start menu every single time you start playback; the app simply doesn't let you stay full screen in the movie selection screen. And... please don't get me started on the experience of using Netflix on my 21:9 monitor on my primary PC. (The mind boggles.)
Fwiw my HTPC setup is an old yet capable Win10 PC on a new 4k 65", 2500k CPU, 8gb memory (will upgrade this soon), Nvidia 960 4GB GPU, Asus XONAR HDAV 1.3 Deluxe with a 20 year old Rotel / Infinity 5.1 hifi-ish system, using a Logitech TK820 from my couch. It is a very nice setup. I will sometimes run Visual Studio on it for some code war puzzles etc.