Lazarus: React to minimization of the application window

Your Form should provide the event OnFormWindowStateChange that is executed whenever something changes at the window (minimizing, maximizing, go back to the normal size).

In this procedure, you can just ask for the current state and execute your desired code:

procedure TForm1.FormWindowStateChange(Sender: TObject);
begin
  if Form1.WindowState = wsMinimized then ...
  if Form1.WindowState = wsMaximized then ...
  if Form1.WindowState = wsNormal then ...
end;   

If the property WindowState of your form has the value wsMinimized, the window has just been minimized, if the property is wsMaximized, the window has just been maximized.
2015-03-23 at 15:29