I wonder if this is related to 794262. I haven't been able to run on device and investigate 794262 because of 795069.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7ecae82c9410f79e36009422a4c63c31118f978a

commit 7ecae82c9410f79e36009422a4c63c31118f978a
Author: Peng Huang <penghuang@chromium.org>
Date: Thu Jan 11 23:26:23 2018

exo: Fix the touch problem for ARC apps

WindowTargeter::FindTargetInRootWindow() uses the wrong touch event
location to check with the bounds of root window. It causes some touch
events are dispatched to the root window instead of the right target.
This change uses the root_location of touch events to fix the problem.

Bug:  793916 
Test: ARC apps with mushroom and cash
Change-Id: I4468d6e3a4608ac69fafa9236abe16a4905e3260
Reviewed-on: https://chromium-review.googlesource.com/857693
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528810}
[modify] https://crrev.com/7ecae82c9410f79e36009422a4c63c31118f978a/ui/aura/window_event_dispatcher_unittest.cc
[modify] https://crrev.com/7ecae82c9410f79e36009422a4c63c31118f978a/ui/aura/window_targeter.cc
[modify] https://crrev.com/7ecae82c9410f79e36009422a4c63c31118f978a/ui/events/event.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/85b33aeead03e03830689494b2f2aec6a4acc600

commit 85b33aeead03e03830689494b2f2aec6a4acc600
Author: Mike Wasserman <msw@chromium.org>
Date: Thu Aug 16 01:40:48 2018

ws: Fix WindowTargeter touch event location check

Use event.location() + ScreenPosition client in FindTargetInRootWindow:
 - location() is in |root_window| coordinates for the ws and clients
 - root_location() is *not* in |root_window| coordinates for clients:
   - |root_window| is the client-root (the top-level client app window)
   - root_location() is Ash's display-root coords (full display bounds)

Add a WindowTreeTest for event locations and root locations.
Add a WindowTargeterTest for FindTargetInRootWindow and regressions.

Bug:  873763 ,  793916 
Test: KSV (Ctrl-Alt-/), Arc, Mash, 2nd display, etc. touch input works.
Change-Id: I7ae541203181f9fc5c65449a10ca927f90d29d96
Reviewed-on: https://chromium-review.googlesource.com/1176462
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583488}
[modify] https://crrev.com/85b33aeead03e03830689494b2f2aec6a4acc600/services/ui/ws2/window_tree_unittest.cc
[modify] https://crrev.com/85b33aeead03e03830689494b2f2aec6a4acc600/ui/aura/window_targeter.cc
[modify] https://crrev.com/85b33aeead03e03830689494b2f2aec6a4acc600/ui/aura/window_targeter_unittest.cc