nixos/fonts: scale X11 core cursor

Most desktop environments manage the cursor using the Xcursor library
by default; this comes with scalable or multiple-sized cursor themes.
However, when running just a simple WM (twm, bspwm, ...) the cursor
handling is left to the X server, which uses a very simple fixed bitmap
font (this is called a "core" cursor). The font is uncomfortably small
on a high DPI display and must be replaced with a saner default.

Up until recently[1] it used to be possible to change the font on the
xserver command line, however the font name is now hardcoded. It's still
possible to change it, though: here I override the `fontcursormisc`
package and set an alias that points to a vector variant of the original
cursor font. The font size is set to match the standard cursor
dimensions on a 96dpi display. It's not perfect but it's a very simple
and effective solution.

[1]: 56ea4c769c

nixos/modules/config/fonts/fonts.nix

@@ -2,6 +2,52 @@
 
 with lib;
 
+let
+  # A scalable variant of the X11 "core" cursor
+  #
+  # If not running a fancy desktop environment, the cursor is likely set to
+  # the default `cursor.pcf` bitmap font. This is 17px wide, so it's very
+  # small and almost invisible on 4K displays.
+  fontcursormisc_hidpi = pkgs.xorg.fontcursormisc.overrideAttrs (old:
+    let
+      # The scaling constant is 230/96: the scalable `left_ptr` glyph at
+      # about 23 points is rendered as 17px, on a 96dpi display.
+      # Note: the XLFD font size is in decipoints.
+      size = 2.39583 * config.services.xserver.dpi;
+      sizeString = builtins.head (builtins.split "\\." (toString size));
+    in
+    {
+      postInstall = ''
+        alias='cursor -xfree86-cursor-medium-r-normal--0-${sizeString}-0-0-p-0-adobe-fontspecific'
+        echo "$alias" > $out/lib/X11/fonts/Type1/fonts.alias
+      '';
+    });
+
+  hasHidpi =
+    config.hardware.video.hidpi.enable &&
+    config.services.xserver.dpi != null;
+
+  defaultFonts =
+    [ pkgs.dejavu_fonts
+      pkgs.freefont_ttf
+      pkgs.gyre-fonts # TrueType substitutes for standard PostScript fonts
+      pkgs.liberation_ttf
+      pkgs.unifont
+      pkgs.noto-fonts-emoji
+    ];
+
+  defaultXFonts =
+    [ (if hasHidpi then fontcursormisc_hidpi else pkgs.xorg.fontcursormisc)
+      pkgs.xorg.fontmiscmisc
+    ] ++ optionals (config.nixpkgs.config.allowUnfree or false)
+    [ # these are unfree, and will make usage with xserver fail
+      pkgs.xorg.fontbhlucidatypewriter100dpi
+      pkgs.xorg.fontbhlucidatypewriter75dpi
+      pkgs.xorg.fontbh100dpi
+    ];
+
+in
+
 {
   imports = [
     (mkRemovedOptionModule [ "fonts" "enableCoreFonts" ] "Use fonts.fonts = [ pkgs.corefonts ]; instead.")
@@ -32,25 +78,9 @@ with lib;
 
   };
 
-  config = {
+  config = mkMerge [
-
+    { fonts.fonts = mkIf config.fonts.enableDefaultFonts defaultFonts; }
-    fonts.fonts = mkIf config.fonts.enableDefaultFonts
+    { fonts.fonts = mkIf config.services.xserver.enable defaultXFonts; }
-      ([
+  ];
-        pkgs.dejavu_fonts
-        pkgs.freefont_ttf
-        pkgs.gyre-fonts # TrueType substitutes for standard PostScript fonts
-        pkgs.liberation_ttf
-        pkgs.xorg.fontmiscmisc
-        pkgs.xorg.fontcursormisc
-        pkgs.unifont
-        pkgs.noto-fonts-emoji
-      ] ++ lib.optionals (config.nixpkgs.config.allowUnfree or false) [
-        # these are unfree, and will make usage with xserver fail
-        pkgs.xorg.fontbhlucidatypewriter100dpi
-        pkgs.xorg.fontbhlucidatypewriter75dpi
-        pkgs.xorg.fontbh100dpi
-      ]);
-
-  };
 
 }