ADDITIONAL SYSTEM INFORMATION :
Ubuntu 18.04

A DESCRIPTION OF THE PROBLEM :
When managing multiple windows on our OpenJfx application sometimes a window that was first opened pops on the front of a current window. It happens when there is a modal window.

We are always sure to initOwner() with the parent window and then set initModality(Modality.WINDOW_MODAL).


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the code.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The dialog should be on top.
ACTUAL -
The first stage is on top.

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ButtonType;
import javafx.scene.control.Dialog;
import javafx.scene.control.Label;
import javafx.stage.Modality;
import javafx.stage.Stage;
import javafx.stage.Window;

public class HelloFx extends Application {
    @Override
    public void start(Stage primaryStage) {
        
        Scene scene = new Scene(new Label("Primary Stage"), 640, 480);
        primaryStage.setScene(scene);
        primaryStage.show();
        
        new TestStage(primaryStage, "First Window").show();

        TestStage secondStage = new TestStage(primaryStage, "Second Window");
        secondStage.show();

        openLastWindow(secondStage);
    }
    
    void openLastWindow(Window owner) {
        Dialog dialog = new Dialog();
        dialog.initOwner(owner);
        dialog.setContentText("Hello world!");
        dialog.setTitle("Last Dialog");
        dialog.initModality(Modality.WINDOW_MODAL);
        dialog.getDialogPane().getButtonTypes().add(ButtonType.CLOSE);

        dialog.showAndWait();
    }
}

class TestStage extends Stage {
    public TestStage(Window owner, String title) {
        initOwner(owner);
        setTitle(title);
        
        this.setScene(new Scene(new Label("Hello World!"), 400, 400));
    }
}
---------- END SOURCE ----------

FREQUENCY : always