The following popup freezes the JavaFX app on show:

        Popup popup = new Popup() {
            {
                getContent().setAll(StackPaneBuilder.create()
                        .children(
                            ListViewBuilder.create()
                                .build(),
                            PaneBuilder.create()
                                .prefWidth(Double.MAX_VALUE) // Comment out these two lines
                                .prefHeight(Double.MAX_VALUE) // to fix the app
                                .build()
                        )
                        .build());
            }
        };