GTK scaling issue on XFCE - GDK_DPI_SCALE is ignored

[zbinlin]

Environment:

• OS: Arch Linux
• Desktop: XFCE
• Display: HiDPI with Font DPI set to 192 in XFCE settings.

The Bug:
On an XFCE desktop configured for HiDPI (192 DPI), Ghostty does not correctly handle GTK's scaling variables. It appears to process the integer GTK_SCALE variable but completely ignores the fractional GDK_DPI_SCALE variable, leading to incorrect UI scaling.

To Reproduce:

1. On an XFCE desktop with a HiDPI screen (e.g., Font DPI set to 192), set the following environment variables. The intended final scale should be 2 * 0.5 = 1x.

   export GTK_SCALE=2
   export GDK_DPI_SCALE=0.5

2. Launch Ghostty.

Expected Result:
The application should honor both variables, calculate a final scale of 1x, and display at a normal, unscaled size.

Actual Result:
Ghostty renders at a 2x scale, completely ignoring the GDK_DPI_SCALE=0.5 instruction.

[00-kat]

I assume the relevant code for this is here:

ghostty/src/apprt/gtk/Surface.zig

Lines 873 to 884 in 9e45da1

	pub fn getContentScale(self: *const Surface) !apprt.ContentScale {
	const gtk_scale: f32 = scale: {
	const widget = self.gl_area.as(gtk.Widget);
	// Future: detect GTK version 4.12+ and use gdk_surface_get_scale so we
	// can support fractional scaling.
	const scale = widget.getScaleFactor();
	if (scale <= 0) {
	log.warn("gtk_widget_get_scale_factor returned a non-positive number: {}", .{scale});
	break :scale 1.0;
	}
	break :scale @floatFromInt(scale);
	};

Which does seem to have a TODO attached too. (#746 is the PR which added it.)

[pluiedev]

I believe GDK_DPI_SCALE only exists for GTK 3 and not for GTK 4. (The mentions of it disappeared in the GTK 4 docs.) You can try this for yourself by comparing GNOME Terminal (GTK 3) versus GNOME Console or Ptyxis (GTK 4).

I think you need to change the gtk-xft-dpi setting instead...? That's how KDE Plasma does it at least. I personally consider it a huge hack since font DPI is only intended to be used for the "Large Text" accessibility feature, but if you're stuck on X11 there's not many alternatives.