streambuilder is rebuilding again and again when keyboard popup or closes

Flutter calls the build() method every time it wants to change anything in the view, and this happens surprisingly often.

You can pass the stream into the stateless widget

 MyApp({Key key, this.stream}) : super(key: key);

Or build the stream in the initState method if the widget is statefull.

@override
void initState() {
  super.initState();
  post = buildStream();
}