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

Question

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.

Answer 1

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();

}