Question about overridden CListBox

Hi,

This is really a general query, as I got the list box to do what I
want. I'm just wondering can somebody who knows the innards of MFC
fill in some background.

I wanted to make a CListBox that cancels the (single) selection when
you click it again. It seems you have to derive a new class, but it
seems to work nicely when I do this and over-ride OnLButtonUp() as
follows:

void ClickyListBox::OnLButtonUp( UINT nFlags, CPoint point )
{
    static int lastItem = -1;
    BOOL bOutside;
    int nItem = ItemFromPoint( point, bOutside );
    if ( ! bOutside )
    {
        if ( nItem == lastItem )
        {
            SetCurSel( -1 );
            lastItem = -1;
        }
        else
        {
            lastItem = nItem;
        }
    }

    CListBox::OnLButtonUp(nFlags, point);
}

My questions:

I originally thought I should remove the call to the base-class
function, but in fact the ClickListBox acts oddly when I do this. What
is CListBox::OnLButtonUp doing that needs to be done?

Also, will the above work on all versions of Win32, or are there any
subtle message-ordering issues that will cause problems?

TIA for any comments. [Obviously I know that I should change lastItem
to a member variable in case there is going to be more than one
instance of the class.]

- Gerry Quinn