终于懂了：FWinControls子控件的显示是由Windows来管理，而不是由Delphi来管理

procedure TWinControl.PaintHandler(var Message: TWMPaint);
var
  I, Clip, SaveIndex: Integer;
  DC: HDC;
  PS: TPaintStruct;
begin
  DC := Message.DC;
  if DC = 0 then DC := BeginPaint(Handle, PS);
  try
    if FControls = nil then PaintWindow(DC) else
    begin
      SaveIndex := SaveDC(DC);
      Clip := SimpleRegion;
      for I := 0 to FControls.Count - 1 do
        with TControl(FControls[I]) do
          if (Visible or (csDesigning in ComponentState) and
            not (csNoDesignVisible in ControlStyle)) and
            (csOpaque in ControlStyle) then
          begin
            Clip := ExcludeClipRect(DC, Left, Top, Left + Width, Top + Height);
            if Clip = NullRegion then Break;
          end;
      if Clip <> NullRegion then PaintWindow(DC);
      RestoreDC(DC, SaveIndex);
    end;
    PaintControls(DC, nil);
  finally
    if Message.DC = 0 then EndPaint(Handle, PS);
  end;
end;

procedure TWinControl.PaintControls(DC: HDC; First: TControl);
var
  I, Count, SaveIndex: Integer;
  FrameBrush: HBRUSH;
begin
  if DockSite and UseDockManager and (DockManager <> nil) then
    DockManager.PaintSite(DC);
  if FControls <> nil then
  begin
    I := 0;
    if First <> nil then
    begin
      I := FControls.IndexOf(First);
      if I < 0 then I := 0;
    end;
    Count := FControls.Count;
    while I < Count do
    begin
      with TControl(FControls[I]) do
        if (Visible or (csDesigning in ComponentState) and
          not (csNoDesignVisible in ControlStyle)) and
          RectVisible(DC, Rect(Left, Top, Left + Width, Top + Height)) then
        begin
          if csPaintCopy in Self.ControlState then
            Include(FControlState, csPaintCopy);
          SaveIndex := SaveDC(DC);
          MoveWindowOrg(DC, Left, Top);
          IntersectClipRect(DC, 0, 0, Width, Height);
          Perform(WM_PAINT, DC, 0);
          RestoreDC(DC, SaveIndex);
          Exclude(FControlState, csPaintCopy);
        end;
      Inc(I);
    end;
  end;
  if FWinControls <> nil then
    for I := 0 to FWinControls.Count - 1 do
      with TWinControl(FWinControls[I]) do
        if FCtl3D and (csFramed in ControlStyle) and
          (Visible or (csDesigning in ComponentState) and
          not (csNoDesignVisible in ControlStyle)) then
        begin // 只是绘制边框而已
          FrameBrush := CreateSolidBrush(ColorToRGB(clBtnShadow));
          FrameRect(DC, Rect(Left - 1, Top - 1, Left + Width, Top + Height),
            FrameBrush);
          DeleteObject(FrameBrush);
          FrameBrush := CreateSolidBrush(ColorToRGB(clBtnHighlight));
          FrameRect(DC, Rect(Left, Top, Left + Width + 1, Top + Height + 1),
            FrameBrush);
          DeleteObject(FrameBrush);
        end;
end;

procedure TWinControl.UpdateShowing;
var
  ShowControl: Boolean;
  I: Integer;
begin
  ShowControl := (FVisible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and
    not (csReadingState in ControlState);
  if ShowControl then
  begin
    if FHandle = 0 then CreateHandle;
    if FWinControls <> nil then
      for I := 0 to FWinControls.Count - 1 do
        TWinControl(FWinControls[I]).UpdateShowing;
  end;
  if FHandle <> 0 then
    if FShowing <> ShowControl then
    begin
      FShowing := ShowControl;
      try
        Perform(CM_SHOWINGCHANGED, 0, 0);
      except
        FShowing := not ShowControl;
        raise;
      end;
    end;
end;

procedure TWinControl.CMShowingChanged(var Message: TMessage);
const
  ShowFlags: array[Boolean] of Word = (
    SWP_NOSIZE + SWP_NOMOVE + SWP_NOZORDER + SWP_NOACTIVATE + SWP_HIDEWINDOW,
    SWP_NOSIZE + SWP_NOMOVE + SWP_NOZORDER + SWP_NOACTIVATE + SWP_SHOWWINDOW);
begin
  SetWindowPos(FHandle, 0, 0, 0, 0, 0, ShowFlags[FShowing]);
end;

procedure TControl.Repaint;
var
  DC: HDC;
begin
  if (Visible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and (Parent <> nil) and
    Parent.HandleAllocated then
    if csOpaque in ControlStyle then
    begin
      DC := GetDC(Parent.Handle);
      try
        IntersectClipRect(DC, Left, Top, Left + Width, Top + Height);
        Parent.PaintControls(DC, Self);
      finally
        ReleaseDC(Parent.Handle, DC);
      end;
    end else
    begin 
      Invalidate;
      Update;
    end;
end;

procedure TControl.Invalidate;
begin
  InvalidateControl(Visible, csOpaque in ControlStyle);
end;

procedure TControl.InvalidateControl(IsVisible, IsOpaque: Boolean);
var
  Rect: TRect;

  function BackgroundClipped: Boolean;
  var
    R: TRect;
    List: TList;
    I: Integer;
    C: TControl;
  begin
    Result := True;
    List := FParent.FControls;
    I := List.IndexOf(Self);
    while I > 0 do
    begin
      Dec(I);
      C := List[I];
      with C do
        if C.Visible and (csOpaque in ControlStyle) then
        begin
          IntersectRect(R, Rect, BoundsRect);
          if EqualRect(R, Rect) then Exit;
        end;
    end;
    Result := False;
  end;

begin
  if (IsVisible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and (Parent <> nil) and
    Parent.HandleAllocated then
  begin
    Rect := BoundsRect;
    InvalidateRect(Parent.Handle, @Rect, not (IsOpaque or
      (csOpaque in Parent.ControlStyle) or BackgroundClipped)); // API
  end;
end;

procedure TControl.Update;
begin
  if Parent <> nil then Parent.Update;
end;

procedure TWinControl.Update;
begin
  if HandleAllocated then UpdateWindow(FHandle); // API
end;