drastically reduced scrollbar flicker when thumb is being dragged

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/wxUNIVERSAL@8507 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/univ/scrolbar.cpp

@@ -129,7 +129,8 @@ wxScrollBar::~wxScrollBar()
 
 void wxScrollBar::DoSetThumb(int pos)
 {
-    // don't do checks here, we're a private function
+    // don't assert hecks here, we're a private function which is meant to be
+    // called with any args at all
     if ( pos < 0 )
     {
         pos = 0;
@@ -139,6 +140,12 @@ void wxScrollBar::DoSetThumb(int pos)
         pos = m_range - m_thumbSize;
     }
 
+    if ( m_thumbPos == pos )
+    {
+        // nothing changed, avoid refreshes which would provoke flicker
+        return;
+    }
+
     if ( m_thumbPosOld == -1 )
     {
         // remember the old thumb position