internal void Set(Header header)
 {
     Header       = header;
     ResizeHandle = null;
     StateHasChanged();
 }
 internal void Set(ResizeHandle resize_handle)
 {
     Header       = null;
     ResizeHandle = resize_handle;
     StateHasChanged();
 }
 internal void Release()
 {
     Header       = null;
     ResizeHandle = null;
     StateHasChanged();
 }