--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -7023,17 +7023,20 @@ nsWindow::GetCSDSupportLevel() {
             sCSDSupportLevel = CSD_SUPPORT_FLAT;
         } else if (strstr(currentDesktop, "i3") != nullptr) {
             sCSDSupportLevel = CSD_SUPPORT_NONE;
         } else if (strstr(currentDesktop, "MATE") != nullptr) {
             sCSDSupportLevel = CSD_SUPPORT_FLAT;
         } else {
             sCSDSupportLevel = CSD_SUPPORT_NONE;
         }
-    }
+    } else {
+        sCSDSupportLevel = CSD_SUPPORT_NONE;
+    }
+
     return sCSDSupportLevel;
 }
 
 int32_t
 nsWindow::RoundsWidgetCoordinatesTo()
 {
     return GdkScaleFactor();
 }