Since both the libinput event source and the key repeat timer have the
same priority, the order in which both handlers are called is
arbitrary if both sources are ready on the same poll return. This
means that sometimes we generate key repeats when there's already a
real key event queued on libinput that would cancel the repeat timer
if only it was processed before.

One solution would be lowering the repeat timer source priority a
notch lower than the libinput source but that would mean that a steady
stream of events from libinput (e.g. pointer device motion) would
prevent any key repeats to happen.

Instead, we can fix this problem by trying to dispatch libinput from
the key repeat timer and checking if the timer source has been
destroyed before generating more key repeats.

https://bugzilla.gnome.org/show_bug.cgi?id=774989