This input field displays '\n' characters as new lines rather than ^J, and accepts the Return, Tab, and up and down arrow keys. This is for editing multiline text.
This is far from the nirvana of text editors, and is probably only good for small bits of text, 10 lines at most. I think FLTK can be used to write a powerful text editor, but it is not going to be a built-in feature. Powerful text editors in a toolkit are a big source of bloat.
Fl_Multiline_Input::Fl_Multiline_Input(int x, int y, int w, int h, const char *label = 0)
Creates a new Fl_Multiline_Input widget using the given position, size, and label string. The default boxtype is FL_DOWN_BOX.
virtual Fl_Multiline_Input::~Fl_Multiline_Input()
Destroys the widget and any value associated with it.