Description of problem:
When I open a drop-down list or a context menu under Wayland, I sometimes spot that the mouse cursor jumps to a different location for a fraction of a second (seems like 1/10 second or something similarly quick). It is probably a race condition, and it's very hard to spot. But we managed to capture the issue with a 240fps camera and furiously fast clicking :)

The mouse cursor always jumps down in vertical direction, it is never shifted horizontally. I can reproduce this best with short drop-down lists (it's most visible when very little changes on the screen otherwise), so that's why I used Display configuration where there's only a single item in the list (probably a bug, but not important here). I can also reproduce this with Nautilus and a context menu shown with a right click. If I do many right clicks quickly in Nautilus window (in an empty space), occasionally I see the mouse cursor teleport down a bit and then go back. It is not necessary to click quickly, this happens even when you do your regular work and click just once or twice, but clicking quickly of course makes it easier and faster to reproduce this.

It is also worth noting that the mouse cursor flickers when opening and closing the drop-down menu. Our theory is that the mouse cursor is moved every time for a fraction of a second, but only sometimes the monitor refresh rate allows us to see it.

We have reproduced this on two different computers.


Version-Release number of selected component (if applicable):
gnome-shell-3.18.2-2.fc23.x86_64
mutter-3.18.2-1.fc23.x86_64
gtk3-3.18.5-2.fc23.x86_64
kernel-4.2.6-300.fc23.x86_64

How reproducible:
it seems always, but you need to try for some time (half a minute or so) and you have to have a keen eye

Steps to Reproduce:
1. right click in nautilus to produce a context menu, or left click on a drop-down list to open and close it, do this repeatedly in quick succession
2. watch the space below the current cursor position
3. occasionally the cursor is visible somewhere below the currently expected position, in a vertical line


Additional info:
This might be related to this bug, but I'm not completely sure:
https://bugzilla.gnome.org/show_bug.cgi?id=758283