fltk/README.Wayland.txt

README.Wayland.txt - Wayland Platform Support for FLTK
------------------------------------------------------


Contents
========

 1   Introduction

 2   Wayland Support for FLTK
   2.1    Configuration
   2.2    Known Limitations

 3   Preparing Platform Specific Code for the Wayland Platform
   3.1    Handling X11 specific Source Code
   3.2    Handling X11 and Wayland Specific Source Code in the Same App
   3.3    Forcing an FLTK App to Always Use the X11 Backend

 4   Platform Specific Notes
   4.1    Debian and Derivatives (like Ubuntu)
   4.2    Fedora
   4.3    FreeBSD


1 Introduction
==============

Version 1.4 of the FLTK library introduces support of the public FLTK API on
the Wayland platform. It requires a Wayland-equipped OS, namely Linux or FreeBSD.
Pre-existing platform-independent source code for FLTK 1.3.x should build and
run unchanged with FLTK 1.4 and the Wayland platform.
The code has been tested on Debian, Ubuntu, RaspberryPiOS and Fedora with
3 distinct Wayland compositors: mutter (Gnome's compositor), weston, and KDE.
The code has also been tested under FreeBSD and the sway wayland compositor.
CJK text-input methods, as well as dead and compose keys are supported.


2 Wayland Support for FLTK
==========================

On Linux and FreeBSD systems, the FLTK library is by default configured so FLTK apps
do all their windowing through the Wayland protocol, all their graphics with
Cairo or EGL, and all text-drawing with Pango. If no Wayland compositor is
available at run-time, FLTK apps fall back to using X11 for windowing.
Cairo and Pango remain used for graphics and text, respectively.

Environment variable FLTK_BACKEND can be used to control whether Wayland or
X11 is used at run time as follows:
- if FLTK_BACKEND is not defined, Wayland is used when possible, otherwise
  X11 is used;
- if $FLTK_BACKEND equals "wayland", the library stops with error if no
  Wayland compositor is available;
- if $FLTK_BACKEND equals "x11", the library uses X11 even if a Wayland
  compositor is available;
- if $FLTK_BACKEND has another value, the library stops with error.

See also 3.3 below for another way to control whether Wayland or X11 is used.

On pure Wayland systems without the X11 headers and libraries, FLTK can be built
with its Wayland backend only (see below).

 2.1 Configuration
------------------

On Linux and FreeBSD systems equipped with the adequate software packages
(see section 4 below), the default building procedure produces a Wayland/X11
hybrid library. On systems lacking all or part of Wayland-required packages,
the default building procedure produces a X11-based library.

Use -DOPTION_USE_WAYLAND=OFF with CMake or "configure --disable-wayland" to build
FLTK for the X11 library when the default would build for Wayland.

CMake OPTION_WAYLAND_ONLY or "--disable-x11" configure argument can
be used to produce a Wayland-only library which can be useful, e.g., when
cross-compiling for systems that lack X11 headers and libraries.

The FLTK Wayland platform uses a library called libdecor which handles window decorations
(i.e., titlebars, shade). Libdecor is bundled in the FLTK source code and FLTK uses by default
this form of libdecor. CMake OPTION_USE_SYSTEM_LIBDECOR can be turned on to have FLTK
use the system's version of libdecor which is available on recent Linux distributions (e.g.,
Debian Bookworm or more recent in packages libdecor-0-0 and libdecor-0-plugin-1-cairo).

 2.2 Known Limitations
----------------------

* A deliberate design trait of Wayland makes application windows ignorant of their exact
placement on screen. It's possible, though, to position a popup window relatively to
another window. This allows FLTK to properly position menu and tooltip windows. But
Fl_Window::position() has no effect on other top-level windows.

* With Wayland, there is no way to know if a window is currently minimized, nor is there any
way to programmatically unset minimization of a window. Consequently, Fl_Window::show() of
a minimized window has no effect.

* Although the FLTK API to read from and write to the system clipboard is fully functional,
it's currently not possible for an app to be notified of changes to the content of
the system clipboard, that is, Fl::add_clipboard_notify() has no effect.

* Narrow windows with a titlebar are silently forced to be wide enough
for the titlebar to display window buttons and a few letters of the title.

* Text input methods have been tested without any understanding of the writing systems,
so feedback on this subject would be helpful.

* Using OpenGL inside Wayland windows doesn't seem to work on RaspberryPi hardware,
although it works inside X11 windows on the same hardware.


3 Preparing Platform Specific Code for the Wayland Platform
===========================================================

While platform-independent source code prepared for FLTK 1.3 is expected
to be compatible with no change with FLTK 1.4 and the Wayland platform,
X11-specific source code may require some attention.

3.1 Handling X11 specific Source Code
-------------------------------------

If an FLTK 1.4 application contains X11-specific code, execution of this code
in the context of an active Wayland session can produce malfunctions or program crashes.
To ensure that X11-specific code gets called only when an X11 connection is active,
check that function fl_x11_display() returns non-NULL before using any X11-specific
function or variable.

3.2 Handling X11 and Wayland Specific Source Code in the Same App
-----------------------------------------------------------------

The recommended way to prepare and use platform-specific code that would contain
X11-specific and possibly Wayland-specific parts is as follows :

a) Organize platform-specific code as follows¹:

  #include <FL/platform.H>

  #ifdef __APPLE__
     *** macOS-specific code ***
  #elif defined(_WIN32)
     *** Windows-specific code ***
  #else
  #  ifdef FLTK_USE_X11
     *** X11-specific code ***
  #  endif
  #  ifdef FLTK_USE_WAYLAND
     *** Wayland-specific code ***
  #  endif
  #endif

b) Make sure to use distinct names for global variables and functions
in the X11- and the Wayland-specific sections.

c) Check that function fl_x11_display() returns non-NULL before using any X11-specific
function or variable, and that fl_wl_display() returns non-NULL before using any
Wayland-specific function or variable. Make sure that fl_open_display() was called
directly or indirectly before using any such symbol.

¹ To be also compatible with macOS+XQuartz, a slightly different organization is necessary:
  #include <FL/platform.H>
  #if defined(FLTK_USE_X11) || defined(FLTK_USE_WAYLAND)
  #  ifdef FLTK_USE_X11
     *** X11-specific code which can run under Linux/Unix or under macOS+XQuartz ***
  #  endif
  #  ifdef FLTK_USE_WAYLAND
     *** Wayland-specific code ***
  #  endif
  #elif defined(__APPLE__)
     *** macOS-specific code which doesn't run under XQuartz ***
  #elif defined(_WIN32)
     *** Windows-specific code ***
  #endif

3.3 Forcing an FLTK App to Always Use the X11 Backend
-----------------------------------------------------

Alternatively, it is possible to force a program linked to a Wayland-enabled
FLTK library to use X11 in all situations by putting this declaration somewhere
in the source code :
  FL_EXPORT bool fl_disable_wayland = true;
FLTK source code and also X11-specific source code conceived for FLTK 1.3
should run with a Wayland-enabled, FLTK 1.4 library with that single change only.


4 Platform Specific Notes
=========================

The following are notes about building FLTK for the Wayland platform
on the various supported Linux distributions/OS.

4.1 Debian and Derivatives (like Ubuntu, RaspberryPiOS)
-------------------------------------------------------

Under Debian, the Wayland platform requires version 11 (a.k.a. Bullseye) or more recent.
Under Ubuntu, the Wayland platform is known to work with version 20.04 (focal fossa) or
more recent.

These packages are necessary to build the FLTK library, in addition to those listed
in section 2.1 of file README.Unix.txt :
- make
- libpango1.0-dev
- libwayland-dev
- wayland-protocols
- libdbus-1-dev
- libxkbcommon-dev
- libxinerama-dev
- libgtk-3-dev   <== highly recommended, gives windows a GTK-style titlebar
- libglew-dev    <== necessary to use OpenGL version 3 or above
- cmake          <== if you plan to build with CMake
- cmake-qt-gui   <== if you plan to use the GUI of CMake

This package is necessary to run FLTK apps under the Gnome-Wayland desktop:
- gnome-core

These packages allow to run FLTK apps under the KDE/Plasma-Wayland desktop:
- kde-plasma-desktop
- plasma-workspace-wayland

Package installation command: sudo apt-get install <package-name ...>


4.2 Fedora
----------

The Wayland platform is known to work with Fedora version 35.

These packages are necessary to build the FLTK library, in addition to
package groups listed in section 2.2 of file README.Unix.txt :
- autoconf
- wayland-devel
- wayland-protocols-devel
- cairo-devel
- libxkbcommon-devel
- pango-devel
- dbus-devel
- mesa-libGLU-devel
- gtk3-devel   <== highly recommended, gives windows a GTK-style titlebar
- glew-devel   <== necessary to use OpenGL version 3 or above
- cmake        <== if you plan to build with CMake
- cmake-gui    <== if you plan to use the GUI of CMake

Package installation command: sudo yum install <package-name ...>


4.3 FreeBSD
-----------

The Wayland platform is known to work with FreeBSD version 13.1 and the sway compositor.

These packages are necessary to build the FLTK library and the sway compositor:
git autoconf pkgconf xorg urwfonts gnome glew seatd sway dmenu-wayland dmenu evdev-proto

Package installation command: sudo pkg install <package-name ...>