Well, I started off with the Color Picker dialog. When I used the
"response" signal, everything was connected: also the Insert button. So
when I clicked the Insert button, the dialog was closed and no color was
inserted.
There already was an existing signal handler , connecting the
"delete_event" signal to gtk_widget_destroy and to the function
color_picker_not_visible. I changed the signals to "destroy" and now it
all works. But, before I apply this to other windows, is this the right way?