The Node
that NotificationPane
accepts in the constructor is actually the content pane OVER which the notification appears, not the content of the notification itself.
There is however a way to achieve what you asked. From the JavaDocs:
The graphic property represents the Node to show within the popup
notification bar that appears on top of the content that is within the
NotificationPane. Despite the term 'graphic', this can be an
arbitrarily complex scenegraph in its own right.
This means that you can indeed put complex Nodes (even whole trees) inside the notification. As long as the Text/Action properties are null, it will occupy all available space (or up to the preferred/max sizes of the node itself), leaving space for the closing button.