DPI Scaling on NixOS with LightDM + XMonad

Question

I've stripped the xserver configuration on my HiDPI laptop down to just:

• displayManager: lightdm (autologin, no greeter)
• windowManager: xmonad
• desktopManager: none (xterm disabled)

And everything is working fine apart from DPI scaling in some places. Specifically: the cursor size and default tty font are both tiny. Apps - terminal emulators, Firefox, etc - are scaling fine as a result of my X settings (below).

Cursor size, and the slow movement that results, is the biggest pain.

So my question is: why would I see proper scaling in some places and not others? What is responsible for the cursor size and other "core" display stuff?

Stuff I've done/tried:

✓ services.xserver.dpi = 180; (sets xserver flag)

✓ .Xresources settings

Xft.dpi: 180
Xft.autohint: 0
Xft.lcdfilter:  lcddefault
Xft.hintstyle:  hintfull
Xft.hinting: 1
Xft.antialias: 1
Xft.rgba: rgb

✓ export GDK_SCALE=2

✓ export GDK_DPI_SCALE=0.5

✓ export XCURSOR_SIZE=32

✓ export QT_AUTO_SCREEN_SCALE_FACTOR=1

Relevant (?) GH issues:

• nixpkgs#22652

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Using NixOS 18.03

Answer 1

Having struggled with this for a while, here are my conclusions...

Whatever black magic is at play making the mouse pointer do it's thing, it has no concern for DPI. In retrospect this seems kinda obvious. A mouse pointer moving across a screen is "lower-level" than a GUI application, such that DPI per se is irrelevant.

The fix? Bigger pointer icon (64x64 worked for me) and more sensitive movement (libinput Accel Speed in my case).