The documentation of Stage.fullScreen states:

     * However once in full-screen mode, {@code Stage}'s {@code x}, {@code y},
     * {@code width}, and {@code height} variables will continue to represent
     * the non-full-screen position and size of the window; the same for
     * {@code iconified}, {@code resizable}, {@code style}, and {@code
     * opacity}. If changes are made to any of these attributes while in
     * full-screen mode, upon exiting full-screen mode the {@code Stage} will
     * assume those attributes.


This does not correspond to the actual behavior of a Stage, where x, y, width, and height will correspond to the actual position and size of a full-screen window (at least on Windows, haven't tested other platforms). Here's a test application:


import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class FullScreenTest extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        var fullScreenButton = new Button("Toggle fullscreen");
        fullScreenButton.setOnAction(_ -> stage.setFullScreen(!stage.isFullScreen()));

        var setSize1Button = new Button("Set size = 400x200");
        setSize1Button.setOnAction(_ -> {
            stage.setWidth(400);
            stage.setHeight(200);
        });

        var setSize2Button = new Button("Set size = 800x400");
        setSize2Button.setOnAction(_ -> {
            stage.setWidth(800);
            stage.setHeight(400);
        });

        var positionLabel = new Label();
        positionLabel.textProperty().bind(Bindings.createStringBinding(
            () -> String.format("Stage position: %.0f, %.0f", stage.getX(), stage.getY()),
            stage.xProperty(), stage.yProperty()));

        var sizeLabel = new Label();
        sizeLabel.textProperty().bind(Bindings.createStringBinding(
            () -> String.format("Stage size: %.0f x %.0f", stage.getWidth(), stage.getHeight()),
            stage.widthProperty(), stage.heightProperty()));

        var root = new VBox(fullScreenButton, setSize1Button, setSize2Button, positionLabel, sizeLabel);
        stage.setScene(new javafx.scene.Scene(root));
        stage.setWidth(400);
        stage.setHeight(200);
        stage.show();
    }
}