How to hide the scrollbars of a MDI child form

Problem/Question/Abstract:

With Delphi 5, how can I hide the scrollbars on a MDI Form? I tried to set the properties AutoScroll, HorzScrollBar.Visible, VertScrollBar.visible to false but it had no effect.

Answer:

This has no effect since the scrollbars do not belong to the MDI frame window itself, they belong to the client window, which is not a Delphi form. Which means one has to attack the problem on the API level. Since this question has come up so frequently in recent days I have modified a sample based on the stock MDI project to include this feature. The salient parts are quoted below.

Open the main forms unit in the IDE. If you don't have a handler for the OnCreate event, add one. In the handler you do this:


if ClientHandle <> 0 then
begin
  if GetWindowLong(ClientHandle, GWL_USERDATA) <> 0 then
    Exit; {cannot subclass client window, userdata already in use}
  SetWindowLong(ClientHandle, GWL_USERDATA, SetWindowLong(ClientHandle,
    GWL_WNDPROC, integer(@ClientWindowProc)));
end;


Add a new standalone function to the unit, it has to go above the FormCreate method since it is referenced in the statement above:


function ClientWindowProc(wnd: HWND; msg: Cardinal; wparam, lparam: Integer): Integer;
  stdcall;
var
  f: Pointer;
begin
  f := Pointer(GetWindowLong(wnd, GWL_USERDATA));
  case msg of
    WM_NCCALCSIZE:
      begin
        if (GetWindowLong(wnd, GWL_STYLE) and (WS_HSCROLL or WS_VSCROLL)) <> 0 then
          SetWindowLong(wnd, GWL_STYLE, GetWindowLong(wnd, GWL_STYLE)
            and not (WS_HSCROLL or WS_VSCROLL));
      end;
  end;
  Result := CallWindowProc(f, wnd, msg, wparam, lparam);
end;


I clipped this code from a larger project, so let's hope I did not create errors in the process. What this code does is to subclass the client window the API way. It stores the old window function into the GWL_USERDATA field of the window structure since it is needed in the replacement window function, all messages need to be passed on to the old window function. There is only one message of interest in this case (the use of a Case results from the larger project, which handles more than this message): WM_NCCALCSIZE. The window gets this message when Windows tries to hide or show the scrollbars, among other cases. And it arrives *before* there is any painting of the scrollbar. So we can check if the window is going to sprout scrollbars and simply remove the scrollbar styles again.

For the purists: there is no need to undo the subclassing before the form is destroyed since the client window is destroyed before the form object.