ADDITIONAL SYSTEM INFORMATION :
Manjaro Linux on a Lenovo Legion laptop. OpenJDK 21.

A DESCRIPTION OF THE PROBLEM :
I've tested many versions ranging from 11 up to the most recent 23-ea+3, all rendering the same problem. When I open a main window, maximize the window, and then open a dialog and set the modality to either WINDOW_MODAL or APPLICATION_MODAL (for modality NONE the main window remains maximized) - when I run showAndWait() for the dialog, the main window is automatically resized to the non-maximized size (although remaining in the maximized state).

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a class that extends Application
2. Create a class that extends Dialog
3. Create some GUI in both application and dialog classes
4. Maximize the main application window
5. Create a dialog, set initOwner(mainApplication.stage), set initModality(WINDOW_MODAL|APPLICATION_MODAL), run showAndWait()
6. The main application window is resized while remaining in the maximized state

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
When the dialog is opened and when I run showAndWait() the maximized window should remain maximized without resizing.
ACTUAL -
The main application window is resized to the size of the non-maximized window state size, while remaining in the maximized state.

---------- BEGIN SOURCE ----------
package test.application;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.input.KeyCombination;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.stage.Modality;
import javafx.stage.Stage;

public class TestApplication extends Application {

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

    private static class TestDialog extends Dialog<ButtonType> {

        public TestDialog(Stage primaryStage) {
            super();
            initModality(Modality.WINDOW_MODAL);
            initOwner(primaryStage);
            setResizable(false);
            DialogPane dialogPane = getDialogPane();
            dialogPane.setContent(new HBox());
            ButtonType okButton = new ButtonType("Ok", ButtonBar.ButtonData.OK_DONE);
            ButtonType cancelButton = new ButtonType("Cancel", ButtonBar.ButtonData.CANCEL_CLOSE);
            dialogPane.getButtonTypes().addAll(okButton, cancelButton);
            showAndWait().ifPresent(response -> {
                if(response.getButtonData() == ButtonType.OK.getButtonData()) {
                    /* Do something */
                }
            });
        }
    }
    @Override
    public void start(Stage primaryStage) throws Exception {
        BorderPane mainLayout = new BorderPane();
        MenuItem itemOpenDialog = new MenuItem("Open Dialog...");
        itemOpenDialog.setAccelerator(KeyCombination.keyCombination("Ctrl-D"));
        itemOpenDialog.setOnAction(event -> new TestDialog(primaryStage));
        MenuItem itemExit = new MenuItem("Exit");
        Menu menuFile = new Menu("File");
        menuFile.getItems().addAll(
                itemOpenDialog,
                new SeparatorMenuItem(),
                itemExit
        );
        MenuBar menuBar = new MenuBar();
        menuBar.getMenus().addAll(menuFile);
        mainLayout.setTop(menuBar);
        Scene scene = new Scene(mainLayout, 1024, 768);
        primaryStage.setMinWidth(800);
        primaryStage.setMinHeight(600);
        primaryStage.setTitle("Test Application");
        primaryStage.setScene(scene);
        primaryStage.show();
        primaryStage.setMaximized(true);
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Haven't found any workaround, it's an internal event that's triggered and resizes the main window. Debugging so far doesn't show where it happens.

FREQUENCY : always