Comment 96 for bug 256429

Here is a more detailed explanation about this patch, it is not related to pam, but to device input event processing.

At some point xf86-input-evdev was changed to a newer version (2.2.5 to 2.3.1 ?), and they changed the events processing :

    evdev: Only send the events at synchronization time.

    Instead of just posting the button/key press/release events to the
    server as soon as they arrive, add them to an internal queue and post
    them once we receive an EV_SYN synchronization event.

    The motion events are always sent first, followed by the queued events.
    There will be one motion event and possibly many queued button/key
    events posted every EV_SYN event.
    [...]

What happens is the additional key code that was added in previous thinkfinger patch is not sent until a real keystroke happens (when you press enter). The patch here adds a synchronisation event after the key code, so that it is immediately posted.

See the comments from Jon Oberheide and Philipp Gortan (mephinet) in this Gentoo Bug report :
http://bugs.gentoo.org/show_bug.cgi?id=298459