Borderless TForm with drop shadow

Question

I have made a TForm derivative that acts like the drop down part of a combo, or a hint window, or a popup menu - a temporary thing. It has no caption - its BorderStyle is set to

Answer 1

"It works on my computer."

(High-res)

But it is quite funny, for I have a faint memory of making the same conclusion as you make, that is, that CS_DROPSHADOW does not work without the thick, resizable, frame. Are you still running Windows Vista, perhaps?

Answer 2

For making drop shadow to work we have to invoke SystemParametersInfo win32 API with SPI_SETDROPSHADOW parameter, to turn on the entire system's drop shadow effect, for more information, please refer to:

SystemParametersInfo

Answer 3

Found it! Here is the proof:

As you can see, the drop shadow now shows properly over the form.

The problem was one of Z-order. It turns out that the shadow is itself a separate window maintained by Windows itself. In Windows 7, it seems to show the shadow underneath the main window. In order to get it to display properly, one needs to move it up.

A genius called Łukasz Płomiński explained this in a thread in the Embarcadero newsgroup. Here is his code to sort it out:

procedure TForm1.FixSysShadowOrder;

  function FindSysShadowOrderProc(WindowHandle: HWND; // handle to window
    Form: TForm1 // application-defined value, 32-bit
    ): BOOL; stdcall;
  var
    Buffer: array [0 .. 255] of char;
    Rect: TRect;
  begin
    Result := True;
    if IsWindowVisible(WindowHandle) then
    begin
      // this code  search for SysShadow window created for this window.
      GetClassName(WindowHandle, Buffer, 255);
      if 0 <> AnsiStrComp(Buffer, PChar('SysShadow')) then
        Exit;

      GetWindowRect(WindowHandle, Rect);
      if (Rect.Left <> Form.Left) or (Rect.Top <> Form.Top) then
        Exit;

      Form.FSysShadowHandle := WindowHandle;
      // stop enumeration
      Result := False;
    end;
  end;

begin
  if not(csDesigning in ComponentState) and
    ((GetClassLong(Handle, GCL_STYLE) and CS_DROPSHADOW) = CS_DROPSHADOW)
    and IsWindowVisible(Handle) then
  begin
    // for speed, proper SysShadow handle is cached
    if FSysShadowHandle = 0 then
      EnumThreadWindows(GetCurrentThreadID(), @FindSysShadowOrderProc,
        lParam(Self));
    // if SysShadow exists, change its z-order, and place it directly below this window
    if FSysShadowHandle <> 0 then
      SetWindowPos(FSysShadowHandle, Handle, 0, 0, 0, 0,
        SWP_NOACTIVATE or SWP_NOMOVE or SWP_NOOWNERZORDER or SWP_NOSIZE);
  end;
end;

You have to work out when to call FixSysShadowOrder(), because Z orders change, and it won't stay right. Łukasz suggested calling it in an idle routine (for example when updating an Action), and on receipt of WM_WINDOWPOSCHANGED message.