In New File->JavaFX->Empty FXML wizard choose Use Java Controller->Generate new one. The TextField allows only entering a unqualified name; it is not displayed where the class will be created. The class is created always in the same package as the FXML file, but it would be better to allow placing the class in arbitrary package. This is also inconsistent with the "use existing class" choice, where full class name is displayed after the class is selected by file dialog.
Also, the various wizard windows have truncated labels: "FXML Na...",
"Controller...", "CSS File Na..."
resolved as part of Issue #220520