Re: [Qemu-devel] [PATCH] gtk: Keep the pointer within window during input grab

[Cole Robinson <crobinso@redhat.com>]

On 04/11/2014 09:28 AM, Takashi Iwai wrote:
> At Fri, 11 Apr 2014 15:07:34 +0200,
> Gerd Hoffmann wrote:
>  
>> On Di, 2014-04-08 at 14:46 +0200, Takashi Iwai wrote:
>>> The current code shows annoying behavior where the X pointer can move
>>> out of the window during the input grab in the absolute mode.  Due to
>>> this, the pointer in qemu window looks as if frozen until the real
>>> (invisible) X pointer comes back to the window again.
>>>
>>> For avoiding such an unexpected lag, this patch limits the pointer
>>> movement only within the qemu window during the input grab in the
>>> absolute mode.  When the pointer goes out, it's moved back to the
>>> boundary again.
>>
>> At least at X11 level it is possible to simply confine the pointer to a
>> certain window (see "man XGrabPointer").  Then you don't have to warp
>> the pointer.
> 
> I have such a vague memory, too, and tried it before my patch, but
> couldn't manage to make it working.  The man page describes the
> pointer event processing, but it doesn't describe how the pointer
> moves.  It's more than a decade when I played with it, so I really
> forget how these things work...
> 
>>  Warping the pointer in absolute mode has ugly effects (see
>> 2bda66028b4962c36d4eabe2995edab12df93691).
> 
> What exact effect do we see?  The commit log doesn't give much clue,
> and I didn't notice any issue with gtk and qxl, so far.
> 

With an F20 guest at least, qxl + gtk was unusable, as described here:

https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg02678.html

The mouse would just warp off the UI.

However your patch still preserves the do_mouse_set short circuiting when in
absolute mode, so that issue doesn't resurface.

The -7/-3 pixel offset I mention in the above mail might come into play, but I
actually can't verify it with my F20 vm, since initiating a pointer grab in
absolute mode with qxl makes the pointer entirely invisible. Thought it was
maybe https://lists.gnu.org/archive/html/qemu-devel/2013-12/msg01627.html but
that didn't help any.

- Cole