Tested using XFWM4 (X11, part of XFCE) as window manager, no idea if that matters or not, and lwjgl3ify 1.1.17

Window dimensions set to 1920x1080 in prismlauncher (via --width and --height mc arguments) and 854x480 in lwjgl3ify config, persists until window is resized

Dimensions set to 854x480 in prismlauncher and 1920x1080 and decorated = false in lwjgl3ify (does not occur when decorated = true), clicking menu buttons does not work until the window gets resized, which once again fixes the scaling

Likely caused by attempting to handle both window sizing mechanisms, one should be preferred over the other