
If you want to stop FLTK from resizing widgets with the mouse, edit Fl_Widget.cxx and replace damage_resize with this code:

int Fl_Widget::damage_resize(int X, int Y, int W, int H) {
  if (x() == X && y() == Y && w() == W && h() == H) return 0;
  if ((Fl::event() == FL_DRAG || Fl::event() == FL_RELEASE) && Fl::event_button() == FL_PUSH) {
   redraw();
  } else {
   resize(X, Y, W, H);
   redraw();
  }
  return 1;
}
