Why isn't Xwayland just a Wayland client?

Pekka Paalanen ppaalanen at gmail.com
Wed Sep 6 10:37:06 UTC 2017

On Wed, 6 Sep 2017 12:11:52 +0200
Olivier Fourdan <fourdan at gmail.com> wrote:

> Hi,
> On 6 September 2017 at 11:48, Joseph Burt <caseorum at gmail.com> wrote:
> > To be clear, my first look at how the X11 channel is used in practice
> > hasn't yet turned up the justification for its existence. The logic
> > usually seems to be "if X client, send event over X11, else Wayland,"
> > which is redundant. There must be something big, since tacking on a
> > X11 channel is a big protocol extension, but I haven't found any
> > discussion of that design decision. Can anyone point me in the right
> > direction?
> >  
> Positioning, stacking, focus management, decorations, X11 window
> properties, ICCCM, etc. all those things that belong specifically to a X11
> window manager which a Wayland compositor isn't. An X11 client running with
> Xwayland does not become a Wayland client, it's still an X11 client,
> whereas Xwayland itself is a Wayland client.
> I guess one could come up with a X11 window manager specific protocol for
> Wayland so that any compatible X11 window manager could integrate and work
> along with a Wayland compositor, but that would be quite a lot of work, and
> I am not sure about the benefits of such an approach.


I understood the question the opposite way: why there is a X11 WM
integrated in each Wayland compositor instead of integrated into
Xwayland and shared by everyone?

I can think a few possible reasons:

- the big DEs already had a working X11 WM and the migration to Wayland
  evolved from that rather than throwing it out first

- there are no suitable Wayland extensions for much of what one can do
  via X11, yet

- the translation between X11 and Wayland window management protocols
  and concepts is very painful if even possible in general, because of
  the fundamental design difference: X11 is low-level (pure mechanism
  with no context) while Wayland is high-level (intent and context to
  let the server do the right thing)

So it was easier to get Xwayland going by putting the X11 WM in the
Wayland compositor, than trying to create a common X11-Wayland WM
translation that would work for everything.

Things might change in the future once Wayland on the desktop matures,

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