Comment 3 for bug 1528319

In case it helps, X provides a sort-of workaround using the xrandr --scale option. For instance, I have a 3840x2160 HiDPI laptop monitor (eDP1) and an external 1680x1050 monitor (HDMI1) above it. With this setup I can make the text etc size approximately the same on both monitors with this command, which increases the zoom on the eDP1 scale:

xrandr --output eDP1 --scale 0.6x0.6

However, with this configuration the text is a bit blurry on the eDP1 screen and the graphics performance is much worse (glxgears only manages 30 fps instead of what it's trying for, ie 60). (Maybe it might work better if the scaling factor is something like 0.5 so the calculations are easier?)

Additionally, if I try to do it the other way around (use xrandr to shrink the screen on HDMI1 above the eDP1 screen), you can't use a y-scale of anything above 1, or X draws the top of the eDP1 screen at the bottom of the HDMI1 screen as well as at the top of the eDP1 screen.

So the workaround isn't really acceptable; it's better to set the HiDPI display to a lower resolution. Or hope that Mir or Wayland will become stable enough to use some time soon.