Why do modal Delphi forms not receive WM_SYSCOMMAND when the user clicks the task bar button?

In a Delphi (2007) program, running on Windows 8.1, I would like to get notified when the user clicks on the task bar button belonging to my program. So I am trapping the WM_SYSCOMMAND which usually gets send in that case.

This works fine for program's main window.

If a modal window is active (opened with Form2.ShowModal), the same code cannot trap the WM_SYSCOMMAND, neither in the main for nor in the modal form. What is different? And is there any way to change this?

This is the code I have added to both forms:

unit unit1;

interface

type
  TForm1 = class(TForm)
    // [...]
    procedure WMSysCommand(var Msg: TWMSysCommand); message WM_SYSCOMMAND;
  end;

 // [...]

implementation

 // [...]

procedure Tf_dzProgressTest.WMSysCommand(var Msg: TWMSysCommand);
begin
  inherited; // place breakpoint here
end;

 // [...]

end.

I also tried to use Application.OnMessage or a TApplicationEvents component and even overriding the form's WndProc method. Neither could trap WM_SYSCOMMAND while a modal form was active.