When an exception is thrown from AnimationTimer::handle, the JavaFX application freezes. The reason is that the user exception will bubble up into framework code, preventing the normal operation of JavaFX.

Reproducer:

public class FailingAnimationTimer extends Application {
    @Override
    public void start(Stage stage) throws Exception {
        var button = new Button("start timer");
        button.setOnAction(_ -> {
            var timer = new AnimationTimer() {
                @Override
                public void handle(long l) {
                    throw new RuntimeException("foo");
                }
            };

            timer.start();
        });

        var root = new HBox();
        root.getChildren().add(new TextField("test"));
        root.getChildren().add(button);
        stage.setScene(new Scene(root));
        stage.show();
    }
}