Bug 1775017 - Don't use gdk_window_get_root_origin on popups, as those get the wrong coordinates on X11+HiDPI. r=stransky

See the linked GTK MR (which also affects GTK3). This can be reproduced
when running tests like
browser/components/extensions/test/browser/browser_ext_popup_select_in_oopif.js

In an X11 session with a HiDPI screen.

Differential Revision: https://phabricator.services.mozilla.com/D149721

widget/gtk/nsWindow.cpp

@@ -2996,21 +2996,28 @@ void nsWindow::SetFocus(Raise aRaise, mozilla::dom::CallerType aCallerType) {
 }
 
 LayoutDeviceIntRect nsWindow::GetScreenBounds() {
-  LayoutDeviceIntRect rect;
-  if (mContainer) {
-    // use the point including window decorations
-    gint x, y;
-    gdk_window_get_root_origin(gtk_widget_get_window(GTK_WIDGET(mContainer)),
-                               &x, &y);
-    rect.MoveTo(GdkPointToDevicePixels({x, y}));
-  } else {
-    rect.MoveTo(WidgetToScreenOffset());
-  }
+  const LayoutDeviceIntPoint origin = [&] {
+    // XXX Can't we use mGdkWindow here?
+    //
+    // Use the point including window decorations. Don't do this for popups,
+    // because we get wrong coordinates for gtk for override-redirect windows in
+    // HiDPI screens, and those don't have window decorations anyways.
+    //
+    // See https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/4820
+    if (mContainer && mWindowType != eWindowType_popup) {
+      gint x, y;
+      gdk_window_get_root_origin(gtk_widget_get_window(GTK_WIDGET(mContainer)),
+                                 &x, &y);
+      return GdkPointToDevicePixels({x, y});
+    }
+    return WidgetToScreenOffset();
+  }();
+
   // mBounds.Size() is the window bounds, not the window-manager frame
   // bounds (bug 581863).  gdk_window_get_frame_extents would give the
   // frame bounds, but mBounds.Size() is returned here for consistency
   // with Resize.
-  rect.SizeTo(mBounds.Size());
+  const LayoutDeviceIntRect rect(origin, mBounds.Size());
 #if MOZ_LOGGING
   gint scale = GdkCeiledScaleFactor();
   LOG("GetScreenBounds %d,%d -> %d x %d, unscaled %d,%d -> %d x %d\n", rect.x,