I've now tried libdecor-gtk with the KDE compositor which provides titlebars

(Server-Side Decoration, a.k.a.SSD mode).

My conclusion is that libdecor-gtk is not compatible with SSD at this point.

This has several consequences

1) libdecor-cairo is to be used in SSD situation (e.g., with the KDE desktop)

2) libdecor-gtk can be used only as a plug-in loaded by dlopen(), because

it's not possible to link both libdecor-cairo and libdecor-gtk in the same executable

since they define at least once the same symbol. Thus, libdecor-gtk can be used only

with CMake  OPTION_USE_SYSTEM_LIBDECOR turned ON. Thus it can't be a public

option for FLTK users, only something for development of the Wayland platform.

3) Ultimately, it won't be possible to show nice GTK-style FLTK window titlebars

until libdecor-gtk is made compatible with the SSD mode by its author.

Alternatively,  build with OPTION_USE_SYSTEM_LIBDECOR should create

2 shared objects, libdecor-cairo.so and libdecor-gtk.so and environment var

LIBDECOR_PLUGIN_DIR could be used for libfltk to find and dlopen() one of them.

4) I still have to find how to prevent libdecor-gtk.so from being loaded when

the compositor is SSD.