source: NonGTP/glut/FLTK/include/FL/README @ 814

These header files are for back-compatability with fltk1.x.  Do not
use these header files in new programs. Use the ones in the fltk
directory.

Most fltk1.x programs should compile with no changes by using these
header files and linking with the fltk2.0 library.

Known changes that must be done to fltk1.1 code to make them compile:

Widgets inside groups will need their x,y coordinates fixed as they
are now relative to the group, not the window.

FL_BOLD, FL_ITALIC - these symbols used to be added to fonts to get
bold and italic. This no longer works. You should replace them with
the single symbol for the font (like FL_HELVETICA_BOLD in place of
FL_HELVETICA+FL_BOLD) or use font->plus(attributes).

Drawing functions are only roughly emulated. You may need to add casts
to float to get your compiler to locate the correct functions. And if
you offset by the x,y of the widget you need to remove these offsets.

Some new members of widgets may hide global data. For instance "width"
and "height" are now members. You can fix this by putting "::" in
front of the global data names.

If you use the Fl_Browser you are probably going to have trouble. All
the indexes are offset by 1 from fltk1.1, ie the first item has an
index of zero, not 1. Adding items with forward slashes in them will
produce a hierarchy, which is pretty confusing when you don't plan on
it.

Menu and browser callbacks: the widget passed is the item, not the
menu or browser. If you use o->parent() to find what window it is in
you will have to change this, in most cases you can use o->window().