When connecting the Steam Deck to a HiDPI monitor, I would like to have the scaling set to 150%. However, this scaling does also apply to the Steam Deck.
Is there a way to decouple the two scaling percentages in order to have 100% on the Steam Deck’s screen and 150% on the HiDPI monitor? Thanks.
Not easily. You would have to launch the Wayland version of the desktop instead of the X version.
Basically Wayland is a newer graphics stack for Linux that has more support for such things like separate scaling. Unfortunately it is still a little bit buggy with KDE. I guess that's why Valve didn't use that for the desktop.
The Steam Deck probably doesn't even have everything needed to run the desktop with Wayland, so it's probably hard to achieve.
Apologies if this is a dumb suggestion, I only briefly tested it, but couldn't you simply run your display at a lower resolution? The deck would perform better and the interface/game scale would match each display correctly.
EDIT: Ah, maybe you mean the desktop, not the steam deck game interface, in which case things would look pretty bad at a lower resolution.