After many months of work, the wp-fractional-scale-v1 protocol for Wayland is set to be merged imminently for fractional scaling support.
The wp-fractional-scale-v1 protocol allows for clients to communicate fractional scaling of surfaces with the compositor. Paired with the wp_viewport protocol can be used for providing a nice fractional scaling implementation rather than needing integer-based scaling.
There is already code pending for wlroots / Sway, GNOME Mutter, and KDE around supporting this fractional scaling protocol. There is also example client implementations pending for Qt, Rust-based Winit, and others.
After an immense amount of work by many parties, release manager Simon Ser has dropped the “needs [acknowledgements]”, “needs review”, and “needs implementations” tags on this protocol and commented that all of the requirements for this protocol are now met and he intends to merge it in the coming days.
Details via this merge request and in particular And in particular there is the protocol text for fractional-scale-v1 for those wondering how it is laid out.