Sometimes after launch app or after new window creation I can see scene fully filled with solid color.
It happens rarely for me. You can see how it looks like at the left side of screenshot.
Controls still active and responding, issue disappears after resize, maximize.

I can't reproduce it with -Dquantum.multithreaded=false. 

Steps to reproduce:

1 Run app
2 If it is not filled with solid color re-run the app

Sample app:

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.stage.Stage;

public class SolidFill extends Application {

    public static void main(String[] args) {
        Application.launch(args);
    }

    @Override
    public void start(Stage s) throws Exception {
        createStage();
    }

    private void createStage() {
        Stage s = new Stage();
        Button b = new Button("123");
        b.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                System.out.println("123 action");
                createStage();
            }
        });
        s.setScene(new Scene(new Group(b), 100, 100));
        s.show();
    }
}