TWinControl.WMNCPaint对非客户的绘制

procedure TWinControl.WMNCPaint(var Message: TMessage);
const
  InnerStyles: array[TBevelCut] of Integer = (0, BDR_SUNKENINNER, BDR_RAISEDINNER, 0);
  OuterStyles: array[TBevelCut] of Integer = (0, BDR_SUNKENOUTER, BDR_RAISEDOUTER, 0);
  EdgeStyles: array[TBevelKind] of Integer = (0, 0, BF_SOFT, BF_FLAT);
  Ctl3DStyles: array[Boolean] of Integer = (BF_MONO, 0);
var
  DC: HDC;
  RC, RW, SaveRW: TRect;
  EdgeSize: Integer;
  WinStyle: Longint;
begin
  { Get window DC that is clipped to the non-client area }
  if (BevelKind <> bkNone) or (BorderWidth > 0) then
  begin
    DC := GetWindowDC(Handle);
    try
      Windows.GetClientRect(Handle, RC);
      GetWindowRect(Handle, RW);
      MapWindowPoints(0, Handle, RW, 2);
      OffsetRect(RC, -RW.Left, -RW.Top);
      ExcludeClipRect(DC, RC.Left, RC.Top, RC.Right, RC.Bottom);
      { Draw borders in non-client area }
      SaveRW := RW;
      InflateRect(RC, BorderWidth, BorderWidth);
      RW := RC;
      if BevelKind <> bkNone then
      begin
        EdgeSize := 0;
        if BevelInner <> bvNone then Inc(EdgeSize, BevelWidth);
        if BevelOuter <> bvNone then Inc(EdgeSize, BevelWidth);
        with RW do
        begin
          WinStyle := GetWindowLong(Handle, GWL_STYLE);
          if beLeft in BevelEdges then Dec(Left, EdgeSize);
          if beTop in BevelEdges then Dec(Top, EdgeSize);
          if beRight in BevelEdges then Inc(Right, EdgeSize);
          if (WinStyle and WS_VSCROLL) <> 0 then Inc(Right, GetSystemMetrics(SM_CYVSCROLL));
          if beBottom in BevelEdges then Inc(Bottom, EdgeSize);
          if (WinStyle and WS_HSCROLL) <> 0 then Inc(Bottom, GetSystemMetrics(SM_CXHSCROLL));
        end;
        DrawEdge(DC, RW, InnerStyles[BevelInner] or OuterStyles[BevelOuter],
          Byte(BevelEdges) or EdgeStyles[BevelKind] or Ctl3DStyles[Ctl3D] or BF_ADJUST);
      end;
      IntersectClipRect(DC, RW.Left, RW.Top, RW.Right, RW.Bottom);
      RW := SaveRW;
      { Erase parts not drawn }
      OffsetRect(RW, -RW.Left, -RW.Top);
      Windows.FillRect(DC, RW, Brush.Handle);
    finally
      ReleaseDC(Handle, DC);
    end;
  end;

  inherited; // 又发现这句对TWinControl.DefaultHandler的调用

  if ThemeServices.ThemesEnabled and (csNeedsBorderPaint in ControlStyle) then
    ThemeServices.PaintBorder(Self, False);
end;