I've been trying to create a MDI form without those annoying scrollbars when a child form is moved outside main form area and I couldn't find an easy way. Setting the scrollbars to visible := false won't work!
So, I found an example on a newsgroup... yeah! Here I show how to do it.

It's a two step proccess.

Step one : put the code below inside the OnCreate event of the main form.

//--------------------------------
if ClientHandle <> 0 then
begin
        if (not (GetWindowLong(ClientHandle, GWL_USERDATA) <> 0)) then
        begin
                SetWindowLong(
                ClientHandle,
                GWL_USERDATA,
                SetWindowLong(ClientHandle, GWL_WNDPROC, integer
                             (@ClientWindowProc))
                );
        end;
end;
//--------------------------------

Step two: Put this standalone function inside the unit that contains the main form, before the OnCreate event (once OnCreate references to this function).

//--------------------------------
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;
//--------------------------------

That's it!!!

The code was originally posted by Peter Below in a newsgroup (borland.public.delphi.objectpascal). I've made some little changes.