Bug 1726900 - Make sure a window is associated with the tray icon on Windows. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D123916

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

mailnews/base/src/nsMessengerWinIntegration.cpp

@@ -166,6 +166,10 @@ nsresult nsMessengerWinIntegration::HideWindow(nsIBaseWindow* aWindow) {
   aWindow->SetVisibility(false);
   sHiddenWindows.AppendElement(aWindow);
 
+  nsresult rv;
+  rv = CreateIconWindow();
+  NS_ENSURE_SUCCESS(rv, rv);
+
   if (!mTrayIconShown) {
     auto idi = IDI_APPLICATION;
     if (sUnreadCount > 0) {