what does that mean? a google search proved fruitless

I assume they mean "Foreign Function Interface" this is a modern C++ code base, and hence can not be easily wrapped in a C function pointer for consumption by non-C++ languages.

FFI = foreign function interface[0], a way to call functions written in one language (generally C) from another. By "FFI friendly" I think the parent meant that Photon Micro GUI isn't something you can easily use from languages other than C++.

[0]: https://en.wikipedia.org/wiki/Foreign_function_interface

Bummer. Higher level low footprint languages such as Nim and Crystal would benefit a lot from bindings for this GUI. Probably Lua too, but that's a different context and Lua already has Tekui and others.

You can still write Crystal wrappers for it, just not using FFI. This looks like it might be polished enough: https://github.com/Papierkorb/bindgen

The bigger issue is that Crystal really needs first-class Windows support before people will use it for GUIs...