Bug 1614421 [Linux] Use GtkInvisible as a fallback widget, r=jhorak

Differential Revision: https://phabricator.services.mozilla.com/D62412

--HG--
extra : moz-landing-system : lando

widget/gtk/WidgetStyleCache.cpp

@@ -758,11 +758,11 @@ GtkWidget* GetWidget(WidgetNodeType aAppearance) {
   if (!widget) {
     widget = CreateWidget(aAppearance);
     // Some widgets (MOZ_GTK_COMBOBOX_SEPARATOR for instance) may not be
-    // available or implemented. Use GtkWindow as a fallback to avoid
+    // available or implemented. Use GtkInvisible as a fallback to avoid
     // potential crashes.
     if (!widget) {
       NS_WARNING(nsPrintfCString("Missing GtkWidget %d\n", aAppearance).get());
-      widget = CreateWindowWidget();
+      widget = gtk_invisible_new();
     }
     // In GTK versions prior to 3.18, automatic invalidation of style contexts
     // for widgets was delayed until the next resize event.  Gecko however,