iOS: small alphas disregarded

This is my code:

``` java
Shell parentShell = ui.getDisplay().getActiveShell();
final Shell abortion = new Shell(parentShell, SWT.APPLICATION_MODAL);
abortion.setAlpha(2);
abortion.setLayout(new GridLayout());
abortion.setMaximized(true);
Composite touchReceiver = new Composite(abortion, SWT.NONE);
touchReceiver.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true));
touchReceiver.addMouseListener(new MouseAdapter() {
@Override
public void mouseUp(MouseEvent e) {
abortion.dispose();
// more code
}
});
```

These are the problems:
In line 3, small alpha values have the following behaviour:
- If the alpha value is 0, 1, or 2, I don't receive the mouse event.
- Small values of alpha don't have an effent, the shell has always an alpha of about 100