Full Screen Basic Guide

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You can use the entire smartphone screen, with 3 steps. Btw it will only work with ultra wide capable games, i guess.
1) Allow full screen for the steam link android app. In most cases is a 2 step work. In steam link app permissions, allow full screen. In the notch management section of your phone switch from "auto" to "always show notch".
2) create a custom screen resolution on your nvidia/amd control panel. This resolution must coincide with that of your smartphone
3) after launching the game set the custom resolution in the game options

Big picture stays in 16: 9, but the game will go full screen.
Tested with shadow of tomb raider and hitman 3 and it works.