Bug 1573813 - use parent window scale factor for the popup/tooltips; r=stransky

For popup windows with parent we need to get scale factor of the parent
window. Because when the windows are hidden they're not receiving updates
about changing scaling factor. So when moving from normal dpi to
the hidpi monitor the newly opened popup windows will have the same scale.

Differential Revision: https://phabricator.services.mozilla.com/D45649

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

widget/gtk/nsWindow.cpp

@@ -6589,11 +6589,22 @@ void nsWindow::SetDrawsInTitlebar(bool aState) {
 }
 
 gint nsWindow::GdkScaleFactor() {
+  // For popup windows with parent window we need to get scale factor of the
+  // parent window. Otherwise the scale factor of the popup is not updated
+  // during it's hidden.
+  GdkWindow* scaledGdkWindow = mGdkWindow;
+  if (mWindowType == eWindowType_popup && mToplevelParentWindow) {
+    scaledGdkWindow = gtk_widget_get_window(GTK_WIDGET(mToplevelParentWindow));
+    // Fallback for windows which parent has been unrealized.
+    if (!scaledGdkWindow) {
+      scaledGdkWindow = mGdkWindow;
+    }
+  }
   // Available as of GTK 3.10+
   static auto sGdkWindowGetScaleFactorPtr =
       (gint(*)(GdkWindow*))dlsym(RTLD_DEFAULT, "gdk_window_get_scale_factor");
-  if (sGdkWindowGetScaleFactorPtr && mGdkWindow)
-    return (*sGdkWindowGetScaleFactorPtr)(mGdkWindow);
+  if (sGdkWindowGetScaleFactorPtr && scaledGdkWindow)
+    return (*sGdkWindowGetScaleFactorPtr)(scaledGdkWindow);
   return ScreenHelperGTK::GetGTKMonitorScaleFactor();
 }