Automatically toggle between dark and light mode PGTK widgets

* src/gtkutil.h: Declare 'xg_set_gtk_theme_dark_mode',
'xg_update_dark_mode_for_all_displays',
'xg_set_initial_dark_mode'.
* src/gtkutil.c (xg_set_gtk_theme_dark_mode): Toggle PGTK dark mode.
(xg_update_dark_mode_for_all_displays): Toggle PGTK dark mode for all
displays.
(xg_set_initial_dark_mode): Determine PGTK dark mode preference and set.
(xg_create_frame_widgets): Set PGTK dark mode preference for widgets.
(xg_create_frame_outer_widgets): Set PGTK dark mode preference for outer
widgets.
* src/xsettings.h: Declare 'xg_get_system_dark_mode'.
* src/xsettings.c (something_changed_gsettingsCB): Update PGTK dark mode
setting.
(xg_get_system_dark_mode): Determine PGTK dark mode preference.
GSETTINGS_COLOR_SCHEME: Defined and set to "color-scheme".
* etc/NEWS: Document automatic dark mode toggling.
(Bug#79689)

4 files changed, 100 insertions, 0 deletions

diff --git a/src/gtkutil.c b/src/gtkutil.c
index c192102730c..7664e20cc6d 100644
--- a/src/gtkutil.c
+++ b/src/gtkutil.c
@@ -36,6 +36,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 typedef struct x_output xp_output;
 #else
 #define xp pgtk
+#include "xsettings.h"
 typedef struct pgtk_output xp_output;
 #endif
 #include "blockinput.h"
@@ -1450,6 +1451,47 @@ xg_set_widget_bg (struct frame *f, GtkWidget *w, unsigned long pixel)
 #endif
 }
 
+/* Apply dark mode preference to GTK window decorations.  */
+
+#ifdef HAVE_PGTK
+void
+xg_set_gtk_theme_dark_mode (bool dark_mode_p, GtkSettings *settings)
+{
+  g_object_set (settings, "gtk-application-prefer-dark-theme",
+                dark_mode_p ? TRUE : FALSE, NULL);
+}
+#endif
+
+/* Update all frames' dark mode based on system setting.  */
+
+#ifdef HAVE_PGTK
+void
+xg_update_dark_mode_for_all_displays (bool dark_mode_p)
+{
+   struct pgtk_display_info *dpyinfo;
+   for (dpyinfo = x_display_list; dpyinfo; dpyinfo = dpyinfo->next)
+     {
+       GdkScreen *screen
+         = gdk_display_get_default_screen (dpyinfo->gdpy);
+       GtkSettings *settings
+         = gtk_settings_get_for_screen (screen);
+       xg_set_gtk_theme_dark_mode (dark_mode_p, settings);
+     }
+}
+#endif
+
+/* Set initial dark mode for a new frame (called during frame
+ * creation).  */
+
+#ifdef HAVE_PGTK
+void
+xg_set_initial_dark_mode (struct frame *f)
+{
+  bool dark_mode_p = xg_get_system_dark_mode ();
+  xg_update_dark_mode_for_all_displays (dark_mode_p);
+}
+#endif
+
 /* Callback called when the gtk theme changes.
    We notify lisp code so it can fix faces used for region for example.  */
 
@@ -1769,6 +1811,10 @@ xg_create_frame_widgets (struct frame *f)
       }
   }
 
+#ifdef HAVE_PGTK
+  xg_set_initial_dark_mode (f);
+#endif
+
   unblock_input ();
 
   return 1;
@@ -1872,6 +1918,10 @@ xg_create_frame_outer_widgets (struct frame *f)
       }
   }
 
+#ifdef HAVE_PGTK
+  xg_set_initial_dark_mode (f);
+#endif
+
   unblock_input ();
 }
 #endif
diff --git a/src/gtkutil.h b/src/gtkutil.h
index 4aef66fedf4..cabf88da73f 100644
--- a/src/gtkutil.h
+++ b/src/gtkutil.h
@@ -184,6 +184,9 @@ extern bool xg_check_special_colors (struct frame *f,
                                      Emacs_Color *color);
 #ifdef HAVE_PGTK
 extern void xg_create_frame_outer_widgets (struct frame *f);
+extern void xg_set_gtk_theme_dark_mode (bool dark_mode_p, GtkSettings *settings);
+extern void xg_update_dark_mode_for_all_displays (bool dark_mode_p);
+extern void xg_set_initial_dark_mode (struct frame *f);
 #endif
 
 #ifndef HAVE_PGTK
diff --git a/src/xsettings.c b/src/xsettings.c
index 30b5ed258ae..72fd70b4b99 100644
--- a/src/xsettings.c
+++ b/src/xsettings.c
@@ -227,6 +227,7 @@ static cairo_font_options_t *font_options;
 #define GSETTINGS_FONT_ANTIALIASING  "font-antialiasing"
 #define GSETTINGS_FONT_RGBA_ORDER    "font-rgba-order"
 #define GSETTINGS_FONT_HINTING       "font-hinting"
+#define GSETTINGS_COLOR_SCHEME       "color-scheme"
 #endif
 
 /* The single GSettings instance, or NULL if not connected to GSettings.  */
@@ -448,6 +449,25 @@ something_changed_gsettingsCB (GSettings *settings,
       apply_gsettings_font_rgba_order (settings);
       store_font_options_changed ();
     }
+  else if (!strcmp (key, GSETTINGS_COLOR_SCHEME))
+    {
+      if (xg_settings_key_valid_p (settings, GSETTINGS_COLOR_SCHEME))
+        {
+          val = g_settings_get_value (settings, GSETTINGS_COLOR_SCHEME);
+          if (val)
+            {
+              g_variant_ref_sink (val);
+              if (g_variant_is_of_type (val, G_VARIANT_TYPE_STRING))
+                {
+                  const char *color_scheme = g_variant_get_string (val, NULL);
+                  /* Check dark mode preference and update all frames. */
+                  bool dark_mode_p = (strstr (color_scheme, "dark") != NULL);
+                  xg_update_dark_mode_for_all_displays (dark_mode_p);
+                }
+              g_variant_unref (val);
+            }
+        }
+    }
 #endif /* HAVE_PGTK */
 }
 
@@ -1109,6 +1129,32 @@ init_gsettings (void)
 #endif /* HAVE_GSETTINGS */
 }
 
+/* Get current system dark mode state.  */
+
+#ifdef HAVE_PGTK
+bool
+xg_get_system_dark_mode (void)
+{
+  if (gsettings_client && xg_settings_key_valid_p (gsettings_client, GSETTINGS_COLOR_SCHEME))
+    {
+      GVariant *val = g_settings_get_value (gsettings_client, GSETTINGS_COLOR_SCHEME);
+      if (val)
+        {
+          g_variant_ref_sink (val);
+          if (g_variant_is_of_type (val, G_VARIANT_TYPE_STRING))
+            {
+              const char *color_scheme = g_variant_get_string (val, NULL);
+              bool dark_mode_p = (strstr (color_scheme, "dark") != NULL);
+              g_variant_unref (val);
+              return dark_mode_p;
+            }
+          g_variant_unref (val);
+        }
+    }
+  return false;
+}
+#endif
+
 /* Init GConf and read startup values.  */
 
 static void
diff --git a/src/xsettings.h b/src/xsettings.h
index 0234fbf043e..2bb8d0c7379 100644
--- a/src/xsettings.h
+++ b/src/xsettings.h
@@ -45,6 +45,7 @@ extern const char *xsettings_get_system_normal_font (void);
 
 #ifdef HAVE_PGTK
 extern cairo_font_options_t *xsettings_get_font_options (void);
+extern bool xg_get_system_dark_mode (void);
 #endif
 
 #endif /* XSETTINGS_H */