fixes SetFirstItem, Thanks to David Wright

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71414 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/listbox_osx.cpp

@@ -133,6 +133,11 @@ void wxListBox::FreeData()
 
 void wxListBox::DoSetFirstItem(int n)
 {
+    // osx actually only has an implementation for ensuring the visibility of a row, it does so  
+    // by scrolling the minimal amount necessary from the current scrolling position.
+    // in order to get the same behaviour I'd have to make sure first that the last line is visible, 
+    // followed by a scrollRowToVisible for the desired line 
+    GetListPeer()->ListScrollTo( GetCount()-1 );
     GetListPeer()->ListScrollTo( n );
 }