[GTK4] Dialog is too large for set size

[tmssngr]

Describe the bug
For a dialog shell (SWT.DIALOG_TRIM) the set size is used for the client area, but not for the full shell.

To Reproduce

import org.eclipse.swt.*;
import org.eclipse.swt.graphics.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.widgets.*;

public class GTK4TooSmallDialog {

	public static void main(String[] args) {
		final Display display = new Display();

		final Shell shell = new Shell(display);
		shell.setLayout(new GridLayout());

		final Button button = createButton(shell, "Open Dialog");
		button.addListener(SWT.Selection, event -> showDialog(shell));

		shell.setSize(400, 300);
		shell.open();

		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep();
			}
		}

		display.dispose();
	}

	private static void showDialog(Shell parent) {
		final Shell shell = new Shell(parent, SWT.DIALOG_TRIM | SWT.PRIMARY_MODAL);
		shell.setLayout(new GridLayout(2, true));

		new Label(shell, SWT.NONE).setText("Label:");

		new Text(shell, SWT.BORDER);

		createButton(shell, "OK").addListener(SWT.Selection, event -> shell.dispose());
		createButton(shell, "Cancel").addListener(SWT.Selection, event -> shell.dispose());

		final Point minSize = shell.computeSize(SWT.DEFAULT, SWT.DEFAULT);
		System.out.println("minSize = " + minSize);
		shell.setSize(minSize);
		shell.open();
	}

	private static Button createButton(Composite parent, String text) {
		final Button button = new Button(parent, SWT.PUSH);
		button.setText(text);
		button.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, true, false));
		return button;
	}
}

• run with SWT_GTK4=1
• click the "Open Dialog" button
• the occurring dialog is too large (by the title bar height)
• the printed, calculated minimum size is 177, 117 which matches the size of the shell's client area
• this bug is not reproducible for SHELL_TRIM

Expected behavior
The dialog shell should have the correct size.

Screenshots
With SWT_GTK4=1 (too large height):

With SWT_GTK4=0 (correct):

Environment:

1. Select the platform(s) on which the behavior is seen:

• • All OS
• • Windows
• • Linux
• • macOS

2. Additional OS info (e.g. OS version, Linux Desktop, etc)
   Ubuntu 26.04