Check the Xft/DPI XSETTINGS

Similar to Xrm's "Xft.dpi" key, but stores in 1024th increments.

src/video/x11/SDL_x11modes.c

@@ -231,9 +231,18 @@ static float GetGlobalContentScale(SDL_VideoDevice *_this)
             }
         }
 
-        /* If that failed, try the XSETTINGS key... */
+        /* If that failed, try the XSETTINGS keys... */
         if (scale_factor <= 0.0) {
             scale_factor = X11_GetXsettingsIntKey(_this, "Gdk/WindowScalingFactor", -1);
+
+            /* The Xft/DPI key is stored in increments of 1024th */
+            if (scale_factor <= 0.0) {
+                int dpi = X11_GetXsettingsIntKey(_this, "Xft/DPI", -1);
+                if (dpi > 0) {
+                    scale_factor = (double) dpi / 1024.0;
+                    scale_factor /= 96.0;
+                }
+            }
         }
 
         /* If that failed, try the GDK_SCALE envvar... */

src/video/x11/SDL_x11settings.c

@@ -27,6 +27,7 @@
 #include "SDL_x11settings.h"
 
 #define SDL_XSETTINGS_GDK_WINDOW_SCALING_FACTOR "Gdk/WindowScalingFactor"
+#define SDL_XSETTINGS_XFT_DPI "Xft/DPI"
 
 static void X11_XsettingsNotify(const char *name, XSettingsAction action, XSettingsSetting *setting, void *data)
 {
@@ -34,7 +35,8 @@ static void X11_XsettingsNotify(const char *name, XSettingsAction action, XSetti
     float scale_factor = 1.0;
     int i;
 
-    if (SDL_strcmp(name, SDL_XSETTINGS_GDK_WINDOW_SCALING_FACTOR) != 0) {
+    if (SDL_strcmp(name, SDL_XSETTINGS_GDK_WINDOW_SCALING_FACTOR) != 0 ||
+        SDL_strcmp(name, SDL_XSETTINGS_XFT_DPI) != 0) {
         return;
     }
 
@@ -47,6 +49,9 @@ static void X11_XsettingsNotify(const char *name, XSettingsAction action, XSetti
         SDL_FALLTHROUGH;
     case XSETTINGS_ACTION_CHANGED:
         scale_factor = setting->data.v_int;
+        if (SDL_strcmp(name, SDL_XSETTINGS_XFT_DPI) == 0) {
+            scale_factor = scale_factor / 1024.0f / 96.0f;
+        }
         break;
     case XSETTINGS_ACTION_DELETED:
         scale_factor = 1.0;