From 69a2b9fa17054794723455fbac84beb51290dfa1 Mon Sep 17 00:00:00 2001
From: Daniel Colascione <dancol@dancol.org>
Date: Fri, 30 Jan 2026 12:30:40 -0500
Subject: [PATCH] xsettings: honor GDK DPI scaling values

Some XWayland setups only report DPI changes through
GDK xsettings, so Emacs missed DPI updates there.
Recognize the GDK DPI and scaling settings and use
them to compute the effective DPI.

* src/xsettings.c (parse_settings): Recognize
Gdk/UnscaledDPI and Gdk/WindowScalingFactor.
Use them to compute DPI when present.
---
 src/xsettings.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/xsettings.c b/src/xsettings.c
index 25edce78841..71cd6a9ad6c 100644
--- a/src/xsettings.c
+++ b/src/xsettings.c
@@ -187,6 +187,8 @@ store_tool_bar_style_changed (const char *newstyle,
 #ifndef HAVE_PGTK
 #if defined USE_CAIRO || defined HAVE_XFT
 #define XSETTINGS_FONT_NAME       "Gtk/FontName"
+#define XSETTINGS_GDK_DPI_NAME    "Gdk/UnscaledDPI"
+#define XSETTINGS_GDK_WSCALE_NAME "Gdk/WindowScalingFactor"
 #endif
 #define XSETTINGS_TOOL_BAR_STYLE  "Gtk/ToolbarStyle"
 #endif
@@ -626,6 +628,15 @@ parse_settings (unsigned char *prop,
   int bytes_parsed = 0;
   int settings_seen = 0;
   int i = 0;
+#if defined USE_CAIRO || defined HAVE_XFT
+  /* Some X environments, e.g. XWayland, communicate DPI changes only
+     through the GDK xsettings values and not the regular Xft one, so
+     recognize both schemes.  We want to see both the GDK window scaling
+     factor and the post-scaling DPI so we can compute our desired
+     actual DPI.  */
+  int gdk_unscaled_dpi = 0;
+  int gdk_window_scale = 0;
+#endif
 
   /* First 4 bytes is a serial number, skip that.  */
 
@@ -668,7 +679,9 @@ parse_settings (unsigned char *prop,
       want_this = strcmp (XSETTINGS_TOOL_BAR_STYLE, name) == 0;
 #if defined USE_CAIRO || defined HAVE_XFT
       if ((nlen > 6 && memcmp (name, "Xft/", 4) == 0)
-	  || strcmp (XSETTINGS_FONT_NAME, name) == 0)
+	  || strcmp (XSETTINGS_FONT_NAME, name) == 0
+	  || strcmp (XSETTINGS_GDK_DPI_NAME, name) == 0
+	  || strcmp (XSETTINGS_GDK_WSCALE_NAME, name) == 0)
 	want_this = true;
 #endif
 
@@ -769,6 +782,10 @@ parse_settings (unsigned char *prop,
               settings->seen |= SEEN_DPI;
               settings->dpi = ival / 1024.0;
             }
+	  else if (strcmp (name, XSETTINGS_GDK_DPI_NAME) == 0)
+	    gdk_unscaled_dpi = ival;
+	  else if (strcmp (name, XSETTINGS_GDK_WSCALE_NAME) == 0)
+	    gdk_window_scale = ival;
           else if (strcmp (name, "Xft/lcdfilter") == 0)
             {
               settings->seen |= SEEN_LCDFILTER;
@@ -786,6 +803,19 @@ parse_settings (unsigned char *prop,
         }
     }
 
+#if defined USE_CAIRO || defined HAVE_XFT
+  if (gdk_unscaled_dpi > 0 && gdk_window_scale > 0)
+    {
+      /* Override any previous DPI settings.  GDK ones are intended to
+	 be authoritative.
+	 See
+	 https://mail.gnome.org/archives/commits-list/2013-June/msg06726.html
+       */
+      settings->seen |= SEEN_DPI;
+      settings->dpi = gdk_window_scale * gdk_unscaled_dpi / 1024.0;
+    }
+#endif
+
   return settings_seen;
 }
 #endif