Most libinput events provide a timestamp in millisecond and/or microsecond resolution. These timestamp usually increase monotonically, but libinput does not guarantee that this always the case. In other words, it is possible to receive an event with a timestamp earlier than the previous event.
For example, if a touchpad has Tap-to-click behaviour enabled, a button event may have a lower timestamp than an event from a different device. Tapping requires the use of timeouts to detect multi-finger taps and/or Tap-and-drag.
Consider the following event sequences from a touchpad and a mouse:
Time Touchpad Mouse
t1 finger down
t2 finger up
t4 tap timeout
For this event sequence, the first event to be sent to a caller is in response to the mouse movement: an event of type LIBINPUT_EVENT_POINTER_MOTION with the timestamp t3. Once the timeout expires at t4, libinput generates an event of LIBINPUT_EVENT_POINTER_BUTTON (press) with a timestamp t1 and an event LIBINPUT_EVENT_POINTER_BUTTON (release) with a timestamp t2.
Thus, the caller gets events with timestamps in the order t3, t1, t2, despite t3 > t2 > t1.