Capture Help Button Click with Custom VCL Style

Question

I have a VCL form that is set for bsDialog with biHelp enabled ("?" icon in application bar). The application is also using a custom VCL Style (Aqua Light Slate).

However I cannot get the WMNCLBUTTONDOWN Windows Message to appear when I click the "?" button. It only works if the VCL Style of the application is changed back to Windows (Default).

procedure TMainFrm.WMNCLButtonDown(var Msg: TWMNCLButtonDown);
begin
  if Msg.HitTest = HTHELP then
  begin
    OutputDebugString('Help button down');
    Msg.Result := 0;
  end
  else
    inherited;
end;

procedure TMainFrm.WMNCLButtonUp(var Msg: TWMNCLButtonUp);
begin
  if Msg.HitTest = HTHELP then
  begin
    OutputDebugString('Help button up');
    Msg.Result := 0;
  end
  else
    inherited;
end;

Is there a way to get these events to fire with a custom VCL style?

Answer 1

The form style hook handles that message:

TFormStyleHook = class(TMouseTrackControlStyleHook)
....
  procedure WMNCLButtonUp(var Message: TWMNCHitMessage); message WM_NCLBUTTONUP;
end;

The implementation includes this

else if (Message.HitTest = HTHELP) and (biHelp in Form.BorderIcons) then
  Help;

This calls the virtual Help method of the form style hook. That is implemented like this:

procedure TFormStyleHook.Help;
begin
  SendMessage(Handle, WM_SYSCOMMAND, SC_CONTEXTHELP, 0)
end;

So you could simply listen for WM_SYSCOMMAND and test wParam for SC_CONTEXTHELP. Like this:

type
  TMainFrm = class(TForm)
  protected
    procedure WMSysCommand(var Message: TWMSysCommand); message WM_SYSCOMMAND;
  end;
....
procedure TMainFrm.WMSysCommand(var Message: TWMSysCommand);
begin
  if Message.CmdType = SC_CONTEXTHELP then begin
    OutputDebugString('Help requested');
    Message.Result := 0;
  end else begin
    inherited;
  end;
end;