https://forums.desmume.org/viewtopic.php?id=10415

Index: src/gtk/main.cpp
--- src/gtk/main.cpp.orig
+++ src/gtk/main.cpp
@@ -3131,6 +3131,11 @@ common_gtk_main( class configured_features *my_config)
     pDrawingArea = gtk_drawing_area_new();
     gtk_container_add (GTK_CONTAINER (pVBox), pDrawingArea);
 
+    /* Set a black background color for a usable full-screen mode. */
+    GdkColor color;
+    memset(&color, 0, sizeof(color));
+    gtk_widget_modify_bg(pDrawingArea, GTK_STATE_NORMAL, &color);
+
     gtk_widget_set_events(pDrawingArea,
                           GDK_EXPOSURE_MASK | GDK_LEAVE_NOTIFY_MASK |
                           GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK |