I am able to use touch correctly with --touch-devices + my mouse on my workstation. So this is probably either the device not getting detected, or not being associated correctly with the right display.

I'll take a look.

Touch works at the login screen, but not after sign-in. Does that sound familiar at all? Maybe some pref/setting getting applied that turns it off?

I suspect this regressed with some recent changes to how touch screens are enabled wrt the PowerButtonController. In particular I think maybe https://chromium-review.googlesource.com/640551 is related. See  issue 759846 .

In particular, ash::ShellDelegateMus::SetTouchscreenEnabled() is not implemented, and I think that enabling the touch screen is now dependent on ash power controller initialization.

The good news is that warx@ has a CL out for review that eliminates that ShellDelegate method and it fixes the problem.

https://chromium-review.googlesource.com/c/chromium/src/+/662657

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f1400a36711fe2f8d473a13121fb7498dec412b0

commit f1400a36711fe2f8d473a13121fb7498dec412b0
Author: Qiang Xu <warx@chromium.org>
Date: Thu Sep 14 02:28:30 2017

mash: remove ToggleTouchpad, Get{Set}TouchscreenEnabled from
ShellDelegate

Changes:
- Create ash::TouchDevicesController to host the removed methods from
ShellDelegate and they talk to ui::InputDeviceControllerClient.
- Move kTouchpadEnabled and kTouchscreenEnabled prefs from chrome to ash.
- Add test coverage for user profile kTouchpadEnabled and
kTouchscreenEnabled. They are triggered by debug accelerator. The
touchscreen global enabled source is already tested in
TabletPowerButtonControllerTest.

Bug:  763253 ,  764486 
Test: device test and added test coverage.
Change-Id: Ic2149132fea3107f2eac217163b166545da5d1c9
Reviewed-on: https://chromium-review.googlesource.com/662657
Commit-Queue: Qiang(Joe) Xu <warx@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501846}
[modify] https://crrev.com/f1400a36711fe2f8d473a13121fb7498dec412b0/ash/BUILD.gn
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[add] https://crrev.com/f1400a36711fe2f8d473a13121fb7498dec412b0/ash/touch/touch_devices_controller.h
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[modify] https://crrev.com/f1400a36711fe2f8d473a13121fb7498dec412b0/chrome/browser/chromeos/preferences.cc
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