How can I remove the title bar / undecorate the window in FLTK on Linux?

I have been doing some things with FLTK on Linux lately, and now I've wondered how I can remove the title bar / undecorate the window. The target Operating System is Linux, but it would be preferrable if it runs on wayland as well as on xorg.

There are two functions that can be used: border(int b) and clear_border().

The border(int b) function tells to the window manager to show or not the border: see here the documentation. This can be used during the execution.

The other useful function is clear_border(): calling it before the Fl_Window::show() function makes the window manager hide the border. See here the documentation.

The (simple) code below shows how to use these functions.

#include <FL/Fl.H>
#include <FL/Fl_Double_Window.H>
#include <FL/Fl_Button.H>


void borderHide(Fl_Widget* w, void* p){
    Fl_Double_Window* win = (Fl_Double_Window*) p;
    // When the input of border() is 0 it tells to the window manager to hide the border.
    win->border(0);
}

void borderShow(Fl_Widget* w, void* p){
    Fl_Double_Window* win = (Fl_Double_Window*) p;
    // When the input of border() is nonzero it tells to the window manager to show the border.
    win->border(1);
}

int main(){

    Fl_Double_Window* W = new Fl_Double_Window(200, 200,"Test");
    // Hide the border from the first execution.
    W->clear_border();

    // Button which implements the border() function for showing the border.
    Fl_Button* S = new Fl_Button(80,150,100,30,"Border on");
    S -> callback(borderShow,W);


    // Button which implements the border() function for hiding the border.
    Fl_Button* H = new Fl_Button(80,100,100,30,"Border off");
    H -> callback(borderHide,W);
    W->end();
    
    W->show();
   
    return  Fl::run();

}