class Fl_Multiline_Input : public Fl_Input

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 FL 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(int x,int y,int w,int h,const char *l = 0);