Recently we've discovered some misbehaviors when attempting to unregister top level windows in add-on SDK. Details can be found under bug 820953. I'm also posting comment from ochameau with more details and pointers: (In reply to Alexandre Poirot (:ochameau) from comment #20) > As far as I know there is not particular white nor black listing of hidden > window, > we just avoid them to be registered on windowwatcher and windowmediator. > > We are having issues because RegisterTopLevel shouldn't ever be called for > our window. And calling UnregisterTopLevelWindow isn't enough. > > I think I found what exactly goes wrong. > Because RegisterTopLevelWindow ends up being called for this window, > nsAppStartup receive a xul-window-registered event and call > EnterLastWindowClosingSurvivalArea: > http://mxr.mozilla.org/mozilla-central/source/xpfe/appshell/src/ > nsAppShellService.cpp#594 > http://mxr.mozilla.org/mozilla-central/source/toolkit/components/startup/ > nsAppStartup.cpp#642 > > But as we never call close on our hidden window, *AND SHOULD NOT HAVE TO DO > IT*. > (gozala: I'll repeat again, but we shouldn't not have to do anything special > to make firefox quit. no listening to any quit-application-requested or > xul-window-destroyed.) > The sibling xul-window-destroyed is never dispatched. > http://mxr.mozilla.org/mozilla-central/source/xpfe/appshell/src/nsXULWindow. > cpp#522 > > So that we can workaround this by dispatching this event. > But we really shouldn't. We should either fix bug 565388 or expose > nsAppShellService::JustCreateTopWindow (or a scriptable equivalent) in order > to be able to implement nsAppShellService::CreateHiddenWindowHelper in JS.