Bug 1740897 [X11] Make parent container of D&D popup transparent to avoid flickering, r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D131252

widget/gtk/nsWindow.cpp

@@ -5252,13 +5252,22 @@ void nsWindow::ConfigureGdkWindow() {
   }
 #endif
 
-  if (mIsDragPopup && GdkIsWaylandDisplay()) {
-    GtkWidget* parent = gtk_widget_get_parent(mShell);
-    if (parent) {
-      GtkWidgetDisableUpdates(parent);
+  if (mIsDragPopup) {
+    if (GdkIsWaylandDisplay()) {
+      // Disable painting to the widget on Wayland as we paint directly to the
+      // widget. Wayland compositors does not paint wl_subsurface
+      // of D&D widget.
+      if (GtkWidget* parent = gtk_widget_get_parent(mShell)) {
+        GtkWidgetDisableUpdates(parent);
+      }
+      GtkWidgetDisableUpdates(mShell);
+      GtkWidgetDisableUpdates(GTK_WIDGET(mContainer));
+    } else {
+      // Disable rendering of parent container on X11 to avoid flickering.
+      if (GtkWidget* parent = gtk_widget_get_parent(mShell)) {
+        gtk_window_set_opacity(GTK_WINDOW(parent), 0.0);
+      }
     }
-    GtkWidgetDisableUpdates(mShell);
-    GtkWidgetDisableUpdates(GTK_WIDGET(mContainer));
   }
 
   if (mWindowType == eWindowType_popup) {