TWinControl = class(TControl)
private
  procedure WMNCHitTest(var Message: TWMNCHitTest); message WM_NCHITTEST;
end;

procedure TWinControl.WMNCHitTest(var Message: TWMNCHitTest);
begin
  with Message do
    if (csDesigning in ComponentState) and (FParent <> nil) then
      Result := HTCLIENT
    else
      inherited;
end;

procedure TWinControl.WndProc(var Message: TMessage);
var
  Form: TCustomForm;
begin
  case Message.Msg of
    WM_SETFOCUS:
      begin
        Form := GetParentForm(Self);
        if (Form <> nil) and not Form.SetFocusedControl(Self) then Exit;
      end;
    WM_KILLFOCUS:
      if csFocusing in ControlState then Exit;
    WM_NCHITTEST:
      begin
        inherited WndProc(Message);
        if (Message.Result = HTTRANSPARENT) and (ControlAtPos(ScreenToClient(
          SmallPointToPoint(TWMNCHitTest(Message).Pos)), False) <> nil) then
          Message.Result := HTCLIENT;
        Exit;
      end;
    WM_MOUSEFIRST..WM_MOUSELAST:
      if IsControlMouseMsg(TWMMouse(Message)) then
      begin
        { Check HandleAllocated because IsControlMouseMsg might have freed the
          window if user code executed something like Parent := nil. }
        if (Message.Result = 0) and HandleAllocated then
          DefWindowProc(Handle, Message.Msg, Message.wParam, Message.lParam);
        Exit;
      end;
    WM_KEYFIRST..WM_KEYLAST:
      if Dragging then Exit;
    WM_CANCELMODE:
      if (GetCapture = Handle) and (CaptureControl <> nil) and
        (CaptureControl.Parent = Self) then
        CaptureControl.Perform(WM_CANCELMODE, 0, 0);
  end;
  inherited WndProc(Message);
end;

THintWindow = class(TCustomControl)
private
  procedure WMNCHitTest(var Message: TWMNCHitTest); message WM_NCHITTEST;
end;

procedure THintWindow.WMNCHitTest(var Message: TWMNCHitTest);
begin
  Message.Result := HTTRANSPARENT;
end;

TScrollBox = class(TScrollingWinControl)
private
  procedure WMNCHitTest(var Message: TMessage); message WM_NCHITTEST;
end;

procedure TScrollBox.WMNCHitTest(var Message: TMessage);
begin
  DefaultHandler(Message); // TScrollBox和TScrollingWinControl都没有覆盖DefaultHandler函数，因此它会调用TWinControl.DefaultHandler
end;

procedure TCustomForm.ClientWndProc(var Message: TMessage);

  procedure Default;
  begin
    with Message do
      Result := CallWindowProc(FDefClientProc, ClientHandle, Msg, wParam, lParam);
  end;

  function MaximizedChildren: Boolean;
  var
    I: Integer;
  begin
    for I := 0 to MDIChildCount - 1 do
      if MDIChildren[I].WindowState = wsMaximized then
      begin
        Result := True;
        Exit;
      end;
    Result := False;
  end;

var
  DC: HDC;
  PS: TPaintStruct;
  R: TRect;
begin
  with Message do
    case Msg of
      WM_NCHITTEST:
        begin
          Default;
          if Result = HTCLIENT then Result := HTTRANSPARENT;
        end;
      WM_ERASEBKGND:
        begin
          FillRect(TWMEraseBkGnd(Message).DC, ClientRect, Brush.Handle);
          { Erase the background at the location of an MDI client window }
          if (FormStyle = fsMDIForm) and (FClientHandle <> 0) then
          begin
            Windows.GetClientRect(FClientHandle, R);
            FillRect(TWMEraseBkGnd(Message).DC, R, Brush.Handle);
          end;
          Result := 1;
        end;
      $3F://!
        begin
          Default;
          if FFormStyle = fsMDIForm then
            ShowMDIClientEdge(FClientHandle, (MDIChildCount = 0) or
              not MaximizedChildren);
        end;
      WM_PAINT:
        begin
          DC := TWMPaint(Message).DC;
          if DC = 0 then
            TWMPaint(Message).DC := BeginPaint(ClientHandle, PS);
          try
            if DC = 0 then
            begin
              GetWindowRect(FClientHandle, R);
              R.TopLeft := ScreenToClient(R.TopLeft);
              MoveWindowOrg(TWMPaint(Message).DC, -R.Left, -R.Top);
            end;
            PaintHandler(TWMPaint(Message));
          finally
            if DC = 0 then
              EndPaint(ClientHandle, PS);
          end;
        end;
    else
      Default;
    end;
end;

procedure TScreen.SetCursor(Value: TCursor);
var
  P: TPoint;
  Handle: HWND;
  Code: Longint;
begin
  if Value <> Cursor then
  begin
    FCursor := Value;
    if Value = crDefault then
    begin
      { Reset the cursor to the default by sending a WM_SETCURSOR to the
        window under the cursor }
      GetCursorPos(P);
      Handle := WindowFromPoint(P);
      if (Handle <> 0) and
        (GetWindowThreadProcessId(Handle, nil) = GetCurrentThreadId) then
      begin
        Code := SendMessage(Handle, WM_NCHITTEST, 0, LongInt(PointToSmallPoint(P)));
        SendMessage(Handle, WM_SETCURSOR, Handle, MakeLong(Code, WM_MOUSEMOVE));
        Exit;
      end;
    end;
    Windows.SetCursor(Cursors[Value]);
  end;
  Inc(FCursorCount);
end;

procedure TCustomCombo.ComboWndProc(var Message: TMessage; ComboWnd: HWnd;
  ComboProc: Pointer);
var
  Point: TPoint;
  Form: TCustomForm;
begin
  try
    with Message do
    begin
      case Msg of
        WM_SETFOCUS:
          begin
            Form := GetParentForm(Self);
            if (Form <> nil) and not Form.SetFocusedControl(Self) then Exit;
          end;
        WM_KILLFOCUS:
          if csFocusing in ControlState then Exit;
        WM_NCHITTEST:
          if csDesigning in ComponentState then
          begin
            Result := HTTRANSPARENT;
            Exit;
          end;
        CN_KEYDOWN, CN_CHAR, CN_SYSKEYDOWN, CN_SYSCHAR:
          begin
            WndProc(Message);
            Exit;
          end;
      end;
      Result := CallWindowProc(ComboProc, ComboWnd, Msg, WParam, LParam);
      if (Msg = WM_LBUTTONDBLCLK) and (csDoubleClicks in ControlStyle) then
        DblClick;
    end;
  except
    Application.HandleException(Self);
  end;
end;