You are right Gonzalo, my previous post was inadequate.

Instructions given by the documentation of Fl::compose(int&) apply to the situation

where one creates a new widget that inputs text and wants to support dead keys,

composed characters, and even complex input methods such as those used for CJK.

But what you're asking is more: give text input support to a new FLTK platform.

I recommend you don't code something that would tie together the Vulkan code and your new 

text input widget, and rather work on two fronts:

1) give full text input support to the Vulkan platform;

2) have your text input widget call Fl::compose(), Fl::compose_reset(), fl_set_spot() and fl_reset_spot()

as described in the documentation.

With this, Vulkan should support all sorts of text input in Fl_Input, Fl_Text_Editor and your new widget.

There's no real documentation of what to do on front 1), only comments here and there

in platform-specific code. You may find inspiration in what Wayland does dealing with variables

Fl::compose_state and Fl_Wayland_Screen_Driver::next_marked_length in file Fl_Wayland_Screen_Driver.cxx.

Thus, you would have to implement these member functions:

- Fl_Vulkan_Screen_Driver::compose(int& del)

- Fl_Vulkan_Screen_Driver::compose_reset() 

- Fl_Vulkan_Screen_Driver::reset_spot()

- the Vulkan equivalent of wl_keyboard_key() that runs each time a key is pressed or released.

I also recommend you to use the xkb library that interfaces very cleanly a client program to

keyboard input operations.