I am using ZorinOS Core version on an HP EliteBook and I noticed a few weird visual things with Steam. Whenever I get steam notifications they are cutoff on screen unless I'm in a game, then it's on screen but very small. I have my computer screen scale set to 200%, so I'm not sure if the issue might be tied to Steam not correctly picking up on the scaling settings or not.
Other issue is that the steam overlay is very small, appears as if it is on 100% scale.
You hit the nail on the head in your first post. Fractional Scaling and WINE do not play well together.
I have little experience with this as I used Zorin OS Lite which scales the DPI setting, which works well with WINE (Steam, in my case).
Hopefully others can offer suggestions. Fractional Scaling needs more work in GnuLinux.
Same issue with the notifications being nearly half cut off the screen on the .deb version. I downloaded a game to test the Steam overlay scale but for some reason I couldn't get the steam overlay to show with shift+tab.