When tooltip is shown in the application below, an error message is printed on my Linux box:
Can't create transparent stage, because your screen doesn't support alpha channel. You need to enable XComposite extension.
I probably really don't have XComposite, but I think the error message should not be printed.

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Tooltip;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class FXTest extends Application {

    @Override public void start(Stage stage) {
        Scene scene = new Scene(new Group(new Rectangle(400, 300)));
        Tooltip tt = new Tooltip("hahaha");
        Tooltip.install(scene.getRoot(), tt);

        stage.setScene(scene);
        stage.sizeToScene();
        stage.show();
    }
    
    public static void main(String[] args) {
        launch(args);
    }
}