How to prevent my window from gaining input focus?

Question

I am implementing an AutoComplete, but one that will match text anywhere in the list of enumerated strings—rather than just the beginning of a suggestion as is the behavior of the built-in SHAutoComplete control

For example:

The "popup" window is a TForm that must:

• set CS_DROPSHADOW style during CreateParams (subject to checking SPI_GETDROPSHADOW of course; I'm not a heathen who doesn't pay his taxes)

  Params.WindowClass.Style := Params.WindowClass.Style or CS_DROPSHADOW;
  

• and of course set its WndParent to the form that holds the TEdit (so that the drop-down appears on top of the form it's popping up over)

  Params.WndParent := FWndParent;
  

I have the Up, Down, Return, Escape keyboard navigation working fine. The typist can quickly select the item they want, and the list of suggestions closes.

The form contains a TListView for the actual items (much like CAutoComplete SHAutoComplete class does in Windows).

Clicking takes focus

If the user clicks an item in the drop-down, then using Inspect (the accessiblity tool in the SDK) we can see that focus was given to one of the listitems in the (now hidden) drop-down menu:

You can even see the title-bar indicates that the entire window on longer has focus.

How can I prevent my window from obtaining input focus?

Attempts

Return MA_NOACTIVATE

I tried responding to the WM_MOUSEACTIVATE window message and return MA_NOACTIVATE:

Return code/value	Description
MA_ACTIVATE (1)	Activates the window, and does not discard the mouse message.
MA_ACTIVATEANDEAT (2)	Activates the window, and discards the mouse message.
MA_NOACTIVATE (3)	Does not activate the window, and does not discard the mouse message.
MA_NOACTIVATEANDEAT (4)	Does not activate the window, but discards the mouse message.

Such as:

procedure TfrmAutoComplete.WMMouseActivate(var Msg: TMessage);
begin
    inherited;
    Msg.Result := MA_NOACTIVATE;
end;

But it still gets focus.

SetFocus during WM_ACTIVATE

I tried messing with things that ought not be messed with, and calling SetFocus during WM_ACTIVATE:

procedure TfrmAutoComplete.WMActivate(var Msg: TMessage);
begin
    inherited;
    Msg.Result := MA_NOACTIVATE;

    Windows.SetFocus(FEdit.Handle);
end;

Raymond Chens talks about why this is a bad idea; although i must confess i don't understand:

The dangers of messing with activation when handling a WM_ACTIVATE message

Raymond Chen, August 9th, 2005

This is basically the same thing as The dangers of playing focus games when handling a WM_KILLFOCUS message, just with activation in place of focus. One developer discovered the hard way that if you mess with activation inside your WM_ACTIVATE handler, things get weird. The author noted that if he posted a message and did the work from the posted message, then everything came out okay.

A follow-up to the original message noted that passing the SWP_NOACTIVATE flag to the SetWindowPos function solves the problem. Do you understand why?

And, of course, when the TEdit loses focus, it hides the suggestion drop-down. So there's probably a focus lock-out race going on.

Short Version

Is there a better way?

Bonus Reading (aka Research Effort)

• Preventing a window from gaining focus automatically (using Windows API)
• Notification Window - Preventing the window from ever getting focus

Answer 1

procedure TfrmAutoComplete.WMActivate(var Msg: TWMActivate);
begin
  inherited;

  if Msg.Active = 0 then
  begin
    if Visible then
    begin
      Hide;
    end;
  end
  else
  begin
    SendMessage(FWndParent, WM_NCACTIVATE, 1, 0);
  end;
end;