[GTK] Minimum size of shell is partly ignored

[tmssngr]

Describe the bug
When setting the minimum size of a "normal" (SWT.SHELL_TRIM) shell, the size is not respected correctly. With GTK4, at least the width is respected.

To Reproduce

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

public class GTK4MinSizeTest {

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

		final Shell shell = new Shell(display);
		shell.addListener(SWT.Resize, event -> System.out.println(shell.getSize()));
		final Composite ourControl = new Composite(shell, SWT.NONE);
		ourControl.addListener(SWT.Paint, event -> {
			final Rectangle bounds = ourControl.getBounds();
			event.gc.drawRectangle(1, 1, bounds.width - 3, bounds.height - 3);
			event.gc.drawString(bounds.toString(), 2, 2, true);
		});
		shell.setLayout(new Layout() {
			@Override
			protected Point computeSize(Composite composite, int wHint, int hHint, boolean flushCache) {
				return getPreferredControlSize();
			}

			@Override
			protected void layout(Composite composite, boolean flushCache) {
				final Rectangle clientArea = composite.getClientArea();
				final Point size = getPreferredControlSize();
				ourControl.setBounds(clientArea.x, clientArea.y, size.x, size.y);
			}

			private Point getPreferredControlSize() {
				return new Point(300, 200);
			}
		});

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

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

		display.dispose();
	}
}

• run this snippet with SWT_GTK4=1 -> the shell has the correct initial size
• try to make it smaller -> on my stock Ubuntu 26.04 machine I can reduce the size down to 300, 153
• run this snippet with SWT_GTK4=0 -> the shell has the correct initial size
• try to make it smaller -> on my stock Ubuntu 26.04 machine I can reduce the size down to 248, 103

Expected behavior
The shell must not be made smaller than the configured minimum size.

Screenshots

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