Fixed refresh bug in border preview control when resizing

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75237 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/richtext/richtextborderspage.cpp

@@ -1344,6 +1344,9 @@ void wxRichTextBorderPreviewCtrl::OnPaint(wxPaintEvent& WXUNUSED(event))
 {
     wxPaintDC dc(this);
 
+    dc.SetBackground(*wxWHITE_BRUSH);
+    dc.Clear();
+
     if (m_attributes)
     {
         wxRect rect = GetClientRect();