关于继承自TGraphicControl的控件在位置变动后无法正确显示的问题

我写了一个类继承自TGraphicControl, 在运行时动态生成该控件( 没有经过注册, 只是用标准C++的继承 )
控件显示没问题... 但当在运行时更改其Left/Top属性移动其位置, 则显示不完整, 显示区域被限制在0, 0, Width, Height这样的区域里...
想来是因为Paint里给的Canvas的区域被限制了, 但看了一下TWinControl的PaintControls里的处理又是正常的(根据控件的Left/Top/Width/Height来自动限制), 原Paint函数处理如下:

    TRect rt( Left, Top, Left + Width, Top + Height );



    Canvas->Brush->Color = clWhite;

    Canvas->FrameRect( rt );



    rt.left++;

    rt.top++;

    rt.right--;

    rt.bottom--;

    Canvas->Brush->Color = clBlack;

    Canvas->FrameRect( rt );



    rt.left++;

    rt.top++;

    rt.right--;

    rt.bottom--;

    Canvas->Brush->Color = clWhite;

    Canvas->FrameRect( rt );

于是我又尝试用API来画, DC还是用的Canvas的Handle, 问题依旧.... 所以就直接用其父窗口的整个区域DC去绘制, 结果就OK了...代码如下

    HDC dc = GetDC( Parent->Handle );

    TBrush *pBrush = new TBrush;

    pBrush->Style = bsClear;

    TPen *pPen = new TPen;



    pPen->Color = clWhite;

    SelectObject( dc, pBrush->Handle );

    SelectObject( dc, pPen->Handle );

    Rectangle( dc,

                Left,

                Top,

                Left + Width,

                Top + Height );



    pPen->Color = clBlack;

    SelectObject( dc, pPen->Handle );

    Rectangle( dc,

                Left + 1,

                Top + 1,

                Left + Width - 1,

                Top + Height - 1 );



    pPen->Color = clWhite;

    SelectObject( dc, pPen->Handle );

    Rectangle( dc,

                Left + 2,

                Top + 2,

                Left + Width - 2,

                Top + Height - 2 );



    ReleaseDC( Parent->Handle, dc );

    delete pBrush;

    delete pPen;

希望有朋友帮忙回答下是为什么...