[GTK] Table.getClientArea reports incorrect values

[tmssngr]

Describe the bug
The Table.getClientArea() method reports incorrect data on GTK3. The x position is wrong by one pixel and the height by approximately the table header.

To Reproduce
Launch this snippet on Windows 11 and Linux (for comparison).

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

public class PaintOverTable {

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

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

		final Table table = new Table(shell, SWT.BORDER | SWT.V_SCROLL | SWT.H_SCROLL);
		final TableColumn column = new TableColumn(table, SWT.LEFT);
		column.setText("Column");
		column.setWidth(100);
		table.setHeaderVisible(true);

		table.addListener(SWT.Paint, event -> {
			final Rectangle clientArea = table.getClientArea();
			event.gc.drawRectangle(clientArea);
		});

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

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

		display.dispose();
	}
}

On Linux the right and lower lines are visible.

Expected behavior
The reported rectangle should cover the same area on all supported platforms.

Screenshots
Windows 11:

This is reasonable, because the table header draws over the upper part and the lower and right lines are one pixel outside the visible area.

Ubuntu 26.04 (Wayland):

Notice the left line is invisible, but the right one. And the lower line is largely off by approximately the table header height.

Note, this bug is not about painting. I just used this example to illustrate the incorrect values visually.

Environment:

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

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