After some emails with Peter, we discovered that holding a key on the keyboard is not an effective activity to trigger the DWT lock mechanism.
hardware: Dell XPS 13 9350
OS: Arch Linux
software: libinput 1.1.5
$ uname -a
Linux hostname 4.4.0-4 #1 SMP PREEMPT Tue Jan 26 19:59:05 CET 2016 x86_64 GNU/Linux
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 005: ID 0c45:670c Microdia
Bus 001 Device 004: ID 04f3:20d0 Elan Microelectronics Corp.
Bus 001 Device 003: ID 0a5c:6412 Broadcom Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Author: Peter Hutterer <email@example.com>
Date: Thu Feb 4 08:31:25 2016 +1000
touchpad: while a key is held down, don't disable dwt
I can confirm that it works now. Thanks.