I forked your repository because of Notification component. I looked deeper into your code and I wanted to get Height of POPUP autosized - Notification with couple lines of message. Next I wanted to add some animations when popup is show and disappear. Next thing was to add possibility to show notifications with custom popup life time per POPUP. I implemented this features, but to get this working I used VBox control as a container for notifications. I have issues with Height animation and there is some hacks to get it working.
Please look into my code if you have some time. Maybe you like it.
I gave also some changes to Demo application so run it a look how it it works.