Nothing is set automatically I run a window manager and it starts what I tell it to start. I observed that at present fewer env variables are now required to obtain proper scaling.
Fun fact: zero envvars are needed for HiDPI support on Wayland.
You do possibly need envvars to enable Wayland support though, but the latest releases of Qt6, GTK4, SDL3 etc. are enabling Wayland by default these days so in the future everything will work out of the box. By default.
X actually exposes both the resolution and physical size of displays. This gives you the DPI if you happen to have mastered basic math. I’ve no idea if this is in fact used but your statement NOTHING provides that is trivially disprovable by runing xrandr --verbose.
Did I say XRandR and mixed DPI in my previous comments? Yeah, I think I did. What the Qt applications currently do is choosing the max DPI and sticking with it. There are some nasty side effects, as I will explain below.
You don’t in fact actually even need apps to be aware of different DPI or dynamically adjust you may scale everything up to the exact same DPI and let X scale it down to the physical resolution.
Don’t forget the side effect: GPU demands and/or CPU demands (depending on the renderer) increase… a lot, nearly 2x in some cases. This might not be acceptable in applications like laptops - have you used projectors in college?
Anecdotally speaking, I gained 1 to 2 hours of battery life just by ditching X11, it’s impressive considering my battery life was like 4 to 5 hours back then. Now it’s actually competitive with Windows which usually gets 6 to 7 hours of battery life.
Furthermore, scaling up and down in multiple passes, instead of letting the clients doing it in “one go” and have the compositor scan it directly onto your screen, leads to problems in font rendering because of some antialiasing shenanigans in addition to the power consumption increase. It’s the very reason why Wayland added a fractional_scaling protocol.
Why would I need to develop another X11 I believe I shall go on using this one which already supported high and mixed DPI just fine when Wayland was a steaming pile of shit nobody in their right mind would use.
Apparently the “nobody” includes GTK, Qt, SDL, and all the mainstream DEs (Xfce and Cinnamon included - even they are preparing to add Wayland support). 90% of the programs I use actually support Wayland pretty well. Good job lad, you managed to invalidate your own argument.
Besides that, you still haven’t properly answered the question of mixed DPI: have you seen a properly-scaled-up LoDPI X11 application? It’s a big problem for XWayland developers. See it here. And yes… those developers are (were?) X11 developers. I think they know how unworkable X11 is, more than you do.