Compositor grabs (was: Re: [PATCH] protocol: Add DnD actions)

Carlos Garnacho carlosg at
Fri Apr 17 05:16:41 PDT 2015

Hey Jonas,

This is drifting a bit off the topic of the original thread, better to
spin this off. I'll reply to the DnD bits in another email.

On Fri, Apr 17, 2015 at 9:50 AM, Jonas Ådahl <jadahl at> wrote:
> On Thu, Apr 16, 2015 at 12:55:31PM +0200, Carlos Garnacho wrote:
>> Hey Jonas,
>> On Thu, Apr 16, 2015 at 10:15 AM, Jonas Ådahl <jadahl at> wrote:
>> <snip>
>> >
>> > I'd have to agree on that it doesn't seem like the best thing to let the
>> > compositor choose the preferred action. Having it apply compositor
>> > specific policy given what the keyboard state or similar will probably
>> > never work out very well, given that for example what modifier state
>> > means what type of action is very application dependent.
>> >
>> > On the other hand, I'm not sure we can currently rely on either side
>> > having keyboard focus during the drag. In weston the source will have the
>> > focus because starting the drag was done with a click which gave the
>> > surface keyboard focus implicitly, but what'd happen if the compositor
>> > has keyboard-focus-follows-mouse? We could probably say that drag implies
>> > an implicit grab on another device on the same seat to enforce no
>> > changing of keyboard focus, but not sure that is better.
>> In gtk+/gnome we currently have the following keybindings active during DnD:
>> - Cursor keys move the drag point, modifiers affect speed
>> - Esc key cancels drag
>> - Modifiers alone pick an action from the offered list
>> So ok, the latter is dubious to punt to compositors, but there's
>> basically no other choice with the 2 first ones.
>> More generally, I have the opinion that compositors grabs should
>> behave all consistently, as in:
>> - Ensuring clients reset all input state (we eg. don't cancel ongoing
>> touches when xdg_popup/dnd/... grabs kick in)
> What does "client reset all input state" mean? What state can a client
> reset?

Let's expand on that example, maybe far-streched, but certainly possible:
- I'm manipulating a client window with 2 fingers on the touchscreen
(say zooming an image)
- Any other interaction on the client makes it pop up an xdg_popup
(say a third touch, a key combo, or the pointer)
- Q: what happens with the two first touches?

Ideally, these touches should be "cancelled" on the first surface
(wl_touch.cancelled seems to be per client, not per-surface though),
and stay non-reactive within the grab, so they wouldn't trigger
anything unintended (they're implicitly grabbed on another surface
after all)

Currently, on the weston code, focusing a bit on the all-touch case,
it actually happens the worst that could happen, the xdg_popup touch
grab redirects already started touch sequences to the grabbing surface
right away, and the original surface will be deaf to them of a sudden,
leading to inconsistent state on both the original and the grabbing
surface wrt those touch sequences. The DnD touch grab doesn't fare
much better, it will ignore other touches than the one starting the
drag, so the pre-grab touches would effectively go nowhere, and AFAICS
similar issues arise with pointer grabs vs touch.

With keyboards, it happens likewise, if compositors are to possibly
consume events there, focus should move out of the previous surface.
IMO, any grabbing model that does not redirect all input, nor ensures
a coherent state in clients is calling for trouble...

In the X11 world, this would roughly be a Virtual Core
Pointer+Keyboard grab (not that touch and active grabs are trouble
free in X11, but...), GTK+ for example does grab both devices on every
of those grabbing places wayland/xdg protocols are trying to cater for
(I've even pondered about adding a gdk_device_grab_pair() for years).

I think some consistent model should be devised here, and embedded
into the protocol (docs).

>> - Ensuring the grab affects the routing of all devices/events, and
>> that no client gets partial streams
> You mean that pointer grab should implicitly grab the touch and keyboard
> device as well? What do you mean with partial stream?

See the example above with surfaces receiving not properly
started/finalized series of event for touch sequences, or the
suggestions in the DnD thread about having drag source/dest handle
keyboard events, while the compositor is certainly interested in
handling/consuming some keyboard events too.


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