I've definitely tried that and it has the wrong effect in a lot of cases. The worst is Chromium (via QtWebEngine, via qutebrowser), which seems to ignore things.

No, I'm not using XWayland.

I edited my comment above after you posted -- the Wayland protocol unfortunately does expose the screen size in millimeters to the client. I'm pretty sure Chromium and QT use this, which is why I need to lie to them.

Another question: is there any sort of "Wayland socket proxy"? Like a stupid simple program that offers up a $WAYLAND_DISPLAY socket to a client, opens its own to a server, and then passes all results through unmodified? It would be really easy to hack a change like this into a proxy like that, but I can't seem to find one. Sort of surprising; the simplicity of the Wayland protocol makes it seem like such a thing should exist. Sort of like waypipe, but without the cross-network magic.