Java Code:
import org.eclipse.swt.SWT;
import org.eclipse.swt.graphics.Rectangle;
import org.eclipse.swt.layout.FormAttachment;
import org.eclipse.swt.layout.FormData;
import org.eclipse.swt.layout.FormLayout;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Event;
import org.eclipse.swt.widgets.Listener;
import org.eclipse.swt.widgets.Sash;
import org.eclipse.swt.widgets.Shell;

public class Snippet107 {

  public static void main(String[] args) {
    Display display = new Display();
    final Shell shell = new Shell(display);
    Button button1 = new Button(shell, SWT.PUSH);
    button1.setText("Button 1");
    final Sash sash = new Sash(shell, SWT.VERTICAL);
    Button button2 = new Button(shell, SWT.PUSH);
    button2.setText("Button 2");

    final FormLayout form = new FormLayout();
    shell.setLayout(form);

    FormData button1Data = new FormData();
    button1Data.left = new FormAttachment(0, 0);
    button1Data.right = new FormAttachment(sash, 0);
    button1Data.top = new FormAttachment(0, 0);
    button1Data.bottom = new FormAttachment(100, 0);
    button1.setLayoutData(button1Data);

    final int limit = 20, percent = 50;
    final FormData sashData = new FormData();
    sashData.left = new FormAttachment(percent, 0);
    sashData.top = new FormAttachment(0, 0);
    sashData.bottom = new FormAttachment(100, 0);
    sash.setLayoutData(sashData);
    sash.addListener(SWT.Selection, new Listener() {
      public void handleEvent(Event e) {
        Rectangle sashRect = sash.getBounds();
        Rectangle shellRect = shell.getClientArea();
        int right = shellRect.width - sashRect.width - limit;
        e.x = Math.max(Math.min(e.x, right), limit);
        if (e.x != sashRect.x) {
          sashData.left = new FormAttachment(0, e.x);
          shell.layout();
        }
      }
    });

    FormData button2Data = new FormData();
    button2Data.left = new FormAttachment(sash, 0);
    button2Data.right = new FormAttachment(100, 0);
    button2Data.top = new FormAttachment(0, 0);
    button2Data.bottom = new FormAttachment(100, 0);
    button2.setLayoutData(button2Data);

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