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Question
I'm writing a TCustomControl descendant that has scrollbars (WS_HSCROLL and WS_VSCROLL styles). Could anyone tell how to enable the control to receive WM_xSCROLL messages at design time?

Try to provide the WM_NCHITTEST message. In the handler, check if you are hitting on scrollbars and return HTVSCROLL or HTHSCROLL in the message result. Otherwise call inherited.

    procedure TScrollCheck.WMNCHitTest(var Msg: TWMNCHitTest);
    var
      XPoint: TPoint;
    begin
      XPoint := ClientToScreen(GetClientRect.BottomRight);
      if XPoint.x < Msg.Pos.X then
        Msg.Result := HTVSCROLL
      else
      if XPoint.y < Msg.Pos.Y then
        Msg.Result := HTHSCROLL
      else
        inherited;
    end;