Hi Pierre,

One key information is that the destructor of an Fl_Group also deletes all the group's children :

 see https://www.fltk.org/doc-1.3/classFl__Group.html#a9a71aac1ca586825ff0c790f8f99f9cf

Now, because FLTK widgets are placed in Fl_Window's and Fl_Window's are Fl_Group's,

when a window is deleted, all the widgets it contains are deleted too.

Another key information is that when you click on the close button of a window,

the window's default close operation occurs which is to call Fl_Window::hide().
This operation does not delete the window (nor its children). The benefit of that

is that you can call Fl_Window::show() later and the window and its children will

reappear. If you need to completely delete a window when its close button is clicked,

you have to change the window's callback, with Fl_Window::callback(), and assign it

a new callback function that does delete the window. At that point, all widgets contained

in the window will get deleted too.

Here is the relevant code to change the window's callback :

static void delete_when_closed(Fl_Widget *win, void *data) {

  Fl::delete_widget(win);

}

// elsewhere in the code where the window is created

  Fl_Window *win = ……

  win->callback(delete_when_closed, NULL); // change the window's callback