[maxdamantus]

It's the same on Wayland. The client (usually part of a toolkit like Gtk/Qt) needs to subscribe to notifications [0] from the server so it can decide the raster size of the surface it wants to draw to. Qt does this on X11 by detecting when your window moves to a screen with another DPI and resizing/rescaling.

I guess the "third" program would be something like xrandr, so the Wayland analogue to that would be wlr-randr (for wlroots compositors), or some other DE-specific tool for configuring screen sizes. Again there's no fundamental difference here.

[0] https://wayland.app/protocols/fractional-scale-v1#wp_fractio...