Inconsistent size of application screen

Question

The problem is that my application size is not consistent. Sometimes application size is same as I defined in my code, but most of the time it's just a small piece on my screen. I'm using kubuntu and Intelij. I think that maybe It's a problem with my system settings or Intelij. So maybe someone had a similar experience and can give some advices how to solve this problem. My application code bellow and pictures of my problem.

import com.almasb.fxgl.app.GameApplication;
import com.almasb.fxgl.app.GameSettings;
import com.almasb.fxgl.dsl.FXGL;
import com.almasb.fxgl.entity.Entity;
import com.almasb.fxgl.input.Input;
import com.almasb.fxgl.input.UserAction;
import javafx.scene.input.KeyCode;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.scene.text.Text;
import java.util.Map;

public class main extends GameApplication {
    @Override
    protected void initSettings(GameSettings settings) {
        settings.setWidth(600);
        settings.setHeight(600);
        settings.setTitle("Basic Game App");
        settings.setVersion("0.1");
        settings.setFullScreenAllowed(true);
    }

    @Override
    protected void initInput() {
        Input input = FXGL.getInput();

        input.addAction(new UserAction("Move Right") {
            @Override
            protected void onAction() {
                player.translateX(5); // move right 5 pixels
                FXGL.getGameState().increment("pixelsMoved", +5);
            }
        }, KeyCode.D);

        input.addAction(new UserAction("Move Left") {
            @Override
            protected void onAction() {
                player.translateX(-5); // move left 5 pixels
                FXGL.getGameState().increment("pixelsMoved", +5);
            }
        }, KeyCode.A);



        input.addAction(new UserAction("Move Down") {
            @Override
            protected void onAction() {
                player.translateY(5); // move down 5 pixels
                FXGL.getGameState().increment("pixelsMoved", +5);
            }
        }, KeyCode.S);
    }

    @Override
    protected void initGameVars(Map<String, Object> vars) {
        vars.put("pixelsMoved", 0);
    }

    private Entity player;

    @Override
    protected void initGame() {
        player = FXGL.entityBuilder()
                .at(0, 0)
                .view(new Rectangle(25, 25, Color.BLUE))
                .buildAndAttach();
    }



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