My class MyListView extends from ListView.
I added custom ListView to my fxml like this:
...
<MyListView />
..

If I open the fxml in Scene Builder I have to adjust the classpath. After I added the MyListView class, Scene Builder starts. If I change for instance the orientation property of MyListView, it is rendered correctly in SceneBuilder (horizontal or vertical). If I save the fxml, Scene Builder reports that the file was saved, but it is not. If I choose save as and choose another file name for the fxml, the orientation property is saved for the custom fxml tag corretly.

If I add the classpath entry manual in the fxml before opening the fxml in Scene Builder like this:

<?scenebuilder-classpath-element ../../../target/classes?>

the save works correctly. Unfortunately on every Scene Builder start with the fxml the dialog appear to adjust the classpath. The missing entry is already in the fxml and also visible in the list in the dialog. I have to check the checkbox "Set up classpath" and click button "Apply" without any change of the classpath and Scene Builder starts. I think the classpath dialog should not be shown in this case.