That is to say, I pefer that <code>this._stack</code> is the list of appWindow instances __you are interested in__.
We could have many AppXXXXManagers for different use cases, and I think each manager is responsible to keep their own lists of appWindow instances.
With this you also don't need to ask WindowManager.getAppFrame in sheets_transition, you just need to ask StackManager to give you the reference of appWindow.