Merge pull request #1246 from rbino/linux-xft-dpi

linux: consider Xft.dpi to scale the content

src/apprt/gtk/Surface.zig

@@ -20,6 +20,7 @@ const ClipboardConfirmationWindow = @import("ClipboardConfirmationWindow.zig");
 const inspector = @import("inspector.zig");
 const gtk_key = @import("key.zig");
 const c = @import("c.zig");
+const x11 = @import("x11.zig");
 
 const log = std.log.scoped(.gtk_surface);
 
@@ -660,8 +661,28 @@ pub fn shouldClose(self: *const Surface) bool {
 pub fn getContentScale(self: *const Surface) !apprt.ContentScale {
     // Future: detect GTK version 4.12+ and use gdk_surface_get_scale so we
     // can support fractional scaling.
-    const scale = c.gtk_widget_get_scale_factor(@ptrCast(self.gl_area));
-    return .{ .x = @floatFromInt(scale), .y = @floatFromInt(scale) };
+    const gtk_scale: f32 = @floatFromInt(c.gtk_widget_get_scale_factor(@ptrCast(self.gl_area)));
+
+    // If we are on X11, we also have to scale using Xft.dpi
+    const xft_dpi_scale = if (!x11.is_current_display_server()) 1.0 else xft_scale: {
+        // Here we use GTK to retrieve gtk-xft-dpi, which is Xft.dpi multiplied
+        // by 1024. See https://docs.gtk.org/gtk4/property.Settings.gtk-xft-dpi.html
+        const settings = c.gtk_settings_get_default();
+
+        var value: c.GValue = std.mem.zeroes(c.GValue);
+        defer c.g_value_unset(&value);
+        _ = c.g_value_init(&value, c.G_TYPE_INT);
+        c.g_object_get_property(@ptrCast(@alignCast(settings)), "gtk-xft-dpi", &value);
+        const gtk_xft_dpi = c.g_value_get_int(&value);
+
+        // As noted above Xft.dpi is multiplied by 1024, so we divide by 1024,
+        // then divide by the default value of Xft.dpi (96) to derive a scale
+        const xft_dpi: f32 = @floatFromInt(@divExact(gtk_xft_dpi, 1024));
+        break :xft_scale xft_dpi / 96;
+    };
+
+    const scale = gtk_scale * xft_dpi_scale;
+    return .{ .x = scale, .y = scale };
 }
 
 pub fn getSize(self: *const Surface) !apprt.SurfaceSize {

src/apprt/gtk/x11.zig

@@ -13,6 +13,12 @@ pub fn is_display(display: ?*c.GdkDisplay) bool {
     ) != 0;
 }
 
+/// Returns true if the app is running on X11
+pub fn is_current_display_server() bool {
+    const display = c.gdk_display_get_default();
+    return is_display(display);
+}
+
 pub const Xkb = struct {
     base_event_code: c_int,
     funcs: Funcs,