This comment makes no sense, XCB isn't a toolkit. You might be thinking of something else with a similar acronym.
But anyway, any solution that tells users to only use a small subset of compatible clients is about as disruptive as just switching wholesale to Wayland. It's not the reason anyone is hanging on to the X server.
Yes, I know what it is. Here are some corrections.
1. XCB is a low level binding to the X11 protocol. It doesn't really replace Xlib. Originally that was the intention, but it's non-trivial to take an Xlib program and port it to XCB.
2. XCB doesn't know anything about scaling or even about XRandR, besides the wire protocol. Just because a client uses XCB is no guarantee that it even uses XRandR. It's definitely not a guarantee that it implements scaling.
3. There is no way to use XRandR correctly to allow multiple pitches, because XRandR doesn't have that functionality. All it does is report an estimated size of the monitor.
4. The scaling you're talking about is happening in the client, not in XCB or XRandR, and if it's ever going to work at all it needs to be controlled by some other setting or environment variable. Someone else in the thread posted a big set of those environment variables. Ideally you wouldn't use XRandR at all, there would be another extension.
5. None of the above applies to Qt, because Qt is a toolkit that does (mostly) implement the scaling for you. That's why it was weird you grouped it with XCB.
Like everything else in XCB, those are just stubs autogenerated from the wire protocol definition. The client has to actually implement the protocol semantics including duplicating a lot of logic that's already present in Xlib. There's no performance benefit to this either. For that reason XRandR is probably one of the most useless and least beneficial things to use XCB for. Signed, someone who actually did try this at one point and ended up reverting back to Xlib.
>But, yes, software using Xcb does have to actually do it correctly for it to work,
There isn't a way to do this correctly, it's a hack. The difference is that Qt attempts to do it automatically, compared to XCB that intentionally does nothing for the programmer because it's a low-level binding.
> There isn't a way to do this correctly, it's a hack
Hacks are often the correct way to do things.
JFC this is annoying. I'm telling you "it works for my professional use case on X11 and doesn't on Wayland" and you keep trying to gaslight me about this by saying it's impossible.
This doesn't work on Wayland. It does work on X11. You can stomp your feet until you're blue in the face but that won't change this basic fact.
If (for whatever reason; I'm still mystified why you care about this) you really really think it's important to get me to switch display servers, make it work on Wayland. Otherwise just let me have my preferences which I largely maintain and package.
But anyway, any solution that tells users to only use a small subset of compatible clients is about as disruptive as just switching wholesale to Wayland. It's not the reason anyone is hanging on to the X server.