I made some progress in the meantime, but I still need to verify it looks good.
Sadly because of how x11 works, it’s nearly impossible for an application to change scaling on the fly, with Wayland, this is easier but not yet implemented.
One strategy I was looking into was asking KDE to render everything at 1.5x.
Without an external monitor connected this is ok, resulting in a 2560x1440 effective size, rendered over 3840x2160 at --scale 1x1.
With an external monitor connected, to get 1920x1080 effective size I’d apply a --scale 0.75x0.75, which will have an effect of stretching 2880x1620 rendered pixels, onto the small 4k screen.
On an external 4k monitor, to have an effective size of 3840x2160 with KDE scaling at 1.5, I’d have to render 5760x3240 and run it with --scale 1.5.
The problem I was running into before was that there’s 2 drivers (xorg drivers or ddx-es) for the integrated intel GPU, one is older soon to be deprecated “intel”. Another one which I was getting by default is called “modesetting”.
For some reason, the “modesetting” ddx limits the texture sizes to 8k*8k, which means that without wayland, in xorg, I can only have a screen at most 8192pixels big, and for 1.5x scaling as described above I need 8640.
With the older, deprecated-ish “intel” xorg driver, I get 32kx32k texture sizes, but I only found that out later. I’ve tried 1.3x scaling in kde, it was blurry and kde was freaking out… Still need to try 1.5x
I’ve also been trying to use the proprietary nvidia 375.66 driver… I’ve made stuff work on one screen by throwing together a 2liner that lightdm would run to setup display with xrandr right before starting the session, but I’ve noticed that while idling it was using 7W more than without it (up to 19-20W from a baseline of a about 12-12.5W). I’ve since decided to remove proprietary Nvidia driver, and blacklist nouveau and use acpi_call to turn off the GPU and not use it at all. (It’s a Quadro M1200, pretty ancient anyway).
Now that I figured out how to get 1.5x scaling, I need to try how this scaling looks, and how it works with plugging in, and unplugging monitors, once I get my my usb-c <-> displayport cable.
Looking forward to Wayland making things better one day in this department.