[RFC : xdg_surface_present() - take 2

Sun Mar 8 22:14:38 PDT 2015

"I have some comments I thought I'd share. See the inline replies:"

Hi Jonas, and thanks a lot for your time,

"This rather sound like something startup notification protocol related
than an "I need attention" request, and I think that problem needs to be
tackled separately."

You're right :  it has nothing to do with the present() request in itself .
I mentioned it mostly because the former discussion mentioned possible ways
to cross-use present() with shell environment variables (like X11 does, to
improve the global user experience). But this is probably going too early
too far, and I should not have mentioned it in my first mail.

"As Bill mentioned, clients that happens to do a lot of things before
connecting will have a very short time from that the connection is
established to that a surface is to be mapped. It can even be trivially
abused to steal focus at unexpected times."

In fact, at least in the current implementation, I think  it cannot be
abused because the surface will be shown and focused if it did not exist
before (that's what already happens), and if it did, it won't raise because
the client will not be able to use a serial coming from another client
(cross-client serials are not implemented nor wanted yet, contrary to what
my former paragraph could let guess ; the compositor stores serials
per-client, should also be per-seat as Bill mentioned).

"I don't think it's a good idea to pass serials between clients and assume
they have any effect, and the server should probably not allow such focus
switching between clients."

Agreed ; let's just forget this for now.

"If 0-is-invalid its not formalized, it should not be relied upon IMO,
but I think it is useful to have a non-valid serial."

Agreed ; this is really the biggest caveat, and it would be nice to agree
on a way to formalize it.
>From an implementation POV : I know that Weston never reuses 0 ; Mutter
probably won't because Jasper most likely agrees :-) ; and regarding
Enlightenment, we're following it closely and regulary pushing patches for
it.

Regards.

2015-03-09 2:57 GMT+01:00 Jonas Ådahl <jadahl at gmail.com>:

> On Fri, Feb 27, 2015 at 11:14:19AM +0100, Manuel Bachmann wrote:
> > Hello fellow developers,
> >
> > I just wanted to continue a discussion which occured some time ago, about
> > the eventuality of adding a "xdg_surface_present()" request to XDG-Shell.
> >
> > (For reference, former discussion entry points are here :
> >
> http://lists.freedesktop.org/archives/wayland-devel/2014-July/016078.html
> >
> http://lists.freedesktop.org/archives/wayland-devel/2014-July/016224.html
> >
> http://lists.freedesktop.org/archives/wayland-devel/2014-August/016269.html
> )
> >
> > To summarize : the idea behing "xdg_surface_present()" is that there are
> > some cases where a surface wants to advertise the fact that it has been
> > updated and the user may want to take a look (think of an IRC chat window
> > which gets new messages containing the user's nickname). There are even
> > some corner cases where the surface may want to be raised and focused
> > directly ; but we do not want to request to be abused this way, a client
> > must be prevented from stealing the focus repeatedly. At last, the
> > compositor's shell should have the last word.
>
> I have some comments I thought I'd share. See the inline replies:
>
> >
> > Here are some of the points that were discussed and the outcomes :
> >
> > - Pekka Paalanen pointed out the request name was unclear and suggested
> to
> > use "xdg_surface_needs/wants_attention()" instead. Jasper St. Pierre
> > pointed out that "_NET_WM_STATE_DEMANDS_ATTENTION" already existed in X11
> > and does not do the same thing. We discussed that again yesterday and
> > thought that present() fitted nicely ;
>
> There is also the "The Present Extension" in X11, so is "present" really
> that much better? If the use case is a client wants attention because
> some reason (be it IRC highlight, new message, task finished or
> whatever), shouldn't the request name contain that information some how?
> Isn't what is discussed here even quite similar to
> "_NET_WM_STATE_DEMANDS_ATTENTION"?
>
> >
> > - We want to implement focus stealing prevention : if the user is
> starting
> > a slow app (browser...), gets back to typing a mail while it starts, and
> at
> > last the browser window appears, the focus should stay in the mail window
> > without his keyboard presses going elsewhere ;
>
> This rather sound like something startup notification protocol related
> than an "I need attention" request, and I think that problem needs to be
> tackled separately.
>
> >
> > - Implementing focus stealing prevention between different clients may be
> > easy : just count the delay between the client has been started and its
> > shell surface actually gets mapped, and if it has been too long and the
> > focus is elsewhere, show the surface without focusing it (but with a
> > notification). The notion of "the client has been started" is vague, but
> at
> > worst, we can use the time when the client did its initial connection to
> > the compositor ;
>
> As Bill mentioned, clients that happens to do a lot of things before
> connecting will have a very short time from that the connection is
> established to that a surface is to be mapped. It can even be trivially
> abused to steal focus at unexpected times.
>
> >
> > - Within a same client application, however, it is harder. Think of a
> > browser where you click "new window" but lots of time pass before it
> > appears. The "starting point" is the pointer click event. So the idea
> would
> > be to pass the Wayland serial corresponding to this event :
> > "xdg_surface_present (surface, <SERIAL>)". It the serial has been issued
> > too long ago, do focus stealing prevention.
>
> Serials are quite easy to guess, but for intra-client focus switching it
> might work. I don't think it's a good idea to pass serials between
> clients and assume they have any effect, and the server should probably
> not allow such focus switching between clients.
>
> >
> > This raises the question : how do we say "We have no serial to pass", for
> > the standard case ? We repeatedly suggested 0 ("xdg_surface_present
> > (surface, 0)") because serials are incrementing globals, so "0" to be
> > issued would be very-very unlikely. Should we formalize that somewhere in
> > the protocol ?
>
> If 0-is-invalid its not formalized, it should not be relied upon IMO,
> but I think it is useful to have a non-valid serial.
>
>
> Jonas
>
> >
> > (Having 2 requests, one with serial and one without, has been suggested
> by
> > krh on IRC ; I personally prefer only one request because they would
> serve
> > the same purpose, but why not ? It would get the need for formalization
> out
> > of the way)
> >
> > We also want to secure the request from garbage random serials ; the
> > implementation behavior is that there is only one serial valid for a few
> > seconds, if the surface has not been focused for some time, it will not
> be
> > able to raise itself even if it random()ly finds the formerly "good"
> serial.
> >
> > -----
> > Now the demos ! Here is the latest code :
> >
> > https://github.com/Tarnyko/weston-xdg_surface_present/commits/test
> >
> > A video for the generic "focus stealing prevention" case (delayed start,
> > focus stays in old surface) between different clients :
> >
> > https://www.youtube.com/watch?v=gifjXyPV3X4
> >
> > and within the same client :
> >
> > https://www.youtube.com/watch?v=Xiq1p5AOf1U
> > -----
> >
> > Any thoughts on this ?
> >
> > --
> > Regards,
> >
> >
> >
> > *Manuel BACHMANN Tizen Project VANNES-FR*
>
> > _______________________________________________
> > wayland-devel mailing list
> > wayland-devel at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/wayland-devel
>
>

-- 
Regards,

*Manuel BACHMANN Tizen Project VANNES-FR*
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