Questions on pointer barrier behaviour

[Michael Weiser]

Hello,

I've written a small X client that detects when the pointer hits a
screen edge in order to trigger actions based on where the edge was hit
- the classic hot corner use case. The trigger for doing this yet again
was that all the tools I looked at either polled the pointer position or
subscribed to all pointer motion which irked me on an aesthetic level. ;)

I've now placed non-constraining pointer barriers at all screen edges
using Xfixes and get notified through XInput2 when the pointer hits them
which seems very efficient to me and works quite well. I've noticed a
couple of things though:

1. Since barriers have a 2px hit box[1] I thought I could nicely use the
XI_BarrierHit event to detect that the user had activated a hot corner
or edge and XI_BarrierLeave to learn when the pointer moved away again.
Since I didn't want to risk any interference with normal user experience
and only wanted to get notified anyway I made the barriers
non-constraining. This however meant that I got a leave event for every
hit even though the pointer was pushing at the very edge of the screen
not going anywhere.

Moving the pointer very slowly it can be seen that first there is a hit
event event for the initial hit but then pushing onwards off-screen
produces a leave as if the pointer actually moved beyond the barrier and
out of the hit box even though continually hitting the screen edge. It
snapping back from off-screen then seems to produce another hit event.

This can be reproduced by changing the interactive demo[2] to place the
barrier at the very top or bottom of the screen.

It also happens with a constraining barrier anywhere on the screen when
approaching it along a screen edge (i.e. as if trying to sneak around it
off-screen and being caught :).

2. Moving the barrier away from the screen edge to then track its
crossing and somehow devise whether it was an entry or departure I found
that for non-constraining barriers, I'm not reliably getting events for
every crossing of the barrier. Neither pointer speed nor input device
(trackpad, mouse) seem to be a factor - for some crossings I just get no
event. This also happens with the interactive demo[2] after making the
barrier non-constraining and deactivating the release code.

3. Making a barrier that starts at a screen edge constraining and then
releasing the pointer immediately after receiving the barrier hit event
shows another peculiar behaviour: When moving the pointer against the
barrier along that screen edge so that it continually tries to push
off-screen makes it get stuck at the barrier *while* barrier hit events
keep coming in and being released. It seems as if the release is getting
lost on the way. This can be reproduced with the interactive demo[2].

As said above I've worked around this to my satisfaction but am still
wondering if those behaviours are to be expected or indeed bugs in any
participant of the interaction (my brain being the prime suspect :).

And finally any advice on whether pointer barriers are a good choice for
the hot corner use case in the first place would be highly welcome.

[1] https://who-t.blogspot.com/2012/12/whats-new-in-xi-23-pointer-barrier.html
[2] http://people.freedesktop.org/~whot/xi2-recipes/pointer-barriers-interactive.c
-- 
Thanks,
Michael