WxSashWindow

wxSashWindow::GetEdgeMargin(wxSashEdgePosition edge) isn't documented. Use it when you want to have a child window in the sash that takes up all the space. For example, for a sash window on the left of the frame that contains a control which should take up all the frame but not overlay the sizing border use this sort of code (from Python but the same principle applies):

    def OnSize(self, event):
        w,h = self.GetClientSizeTuple()
        self.tree.SetDimensions(0, 0, w - self.GetEdgeMargin(wxSASH_RIGHT), h)