_firstInViewport = 0;
_lastInViewport = MainListBox.Items.Count - 1;

// determine first visible item
for (int i = 0; i < MainListBox.Items.Count; i++)
{
	ListBoxItem item = (ListBoxItem)MainListBox.ItemContainerGenerator.ContainerFromIndex(i);
	if (item == null || !IsVisibleInViewPort(item, MainListBox))
		continue;

	_firstInViewport = i;
	break;
}

for (int i = _firstInViewport; i < MainListBox.Items.Count; i++)
{
	ListBoxItem item = (ListBoxItem)MainListBox.ItemContainerGenerator.ContainerFromIndex(i);
	if (item == null)
	{
		continue;
	}

	if (!IsVisibleInViewPort(item, MainListBox))
	{
		_lastInViewport = i;
		break;
	}
}