Note @Could be device specific problem 

Could not repro on Peppy device version 8743.25.0/54.0.2840.24

Trapti was able to repro on the same Peppy with 8743.25.0/54.0.2840.24.  Seems to be intermittent.

The UI is the problem here - the touchpad itself works on the Status Tray, just not on the "+ Add person", "Apps", "Select your language" dropdown or a few other UI elements.  Cursor movement is unimpaired.  Keyboard navigation is still working also.

External mouse is having the same problem

@Note It blocks signin first time in Public Session via touchpad or via clicking via mouse....could not change keyboard,language too

glevin@ can you set aside the a11y work you're doing for a bit and dig into this?

Not sure what the root cause is, but there's something broken with the login web content after going through the EE enrollment flow.  None of the web content (user pod or Shut down / Apps / Add person buttons) are getting any mouse messages.  Notice that the mouse pointer doesn't become a finger when hovered over UI elements.  If you restart, or log in and out, this clears the broken web content, and things work fine.  I think the problem is something left over or broken by the (Enterprise?) enrollment.

It almost looks like its trying to load the OOBE content twice.  In comparing js element inspector for the broken case and the "normal" (after restart) case, the broken case is missing the lines

<script src="chrome://oobe/login.js"></script>
<script src="chrome://oobe/gaia_auth_host.js"></script>

but contains duplicate copies of many of the <div> elements that normally appear only once.  Further, in the broken case, the following error appears in the console:

load_time_data.js:139 Unexpected condition on chrome://oobe/oobe: should only include this file once
expect @ load_time_data.js:139

I don't know the code well enough to know what this all means, but I'll keep looking.

Bisect results:

You are probably looking for a change made after 415803 (known good), but no later than 415804 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/4b2ccc21abd7383301000507b1cdbf7696deb659..29276a7a0b4a0d59fa0a420cf9e7885286020bc4

Looks like https://codereview.chromium.org/2254623003

Seen in M55 too.

M	ChromeOS	Chrome	ARC	Type	Channel
55	8829.0.0	55.0.2866.0	3294522	release	dev

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1031e3dc40e1af209cf973ccaf22ea8b3b409122

commit 1031e3dc40e1af209cf973ccaf22ea8b3b409122
Author: glevin <glevin@chromium.org>
Date: Sat Sep 24 05:49:12 2016

Fix clicks on Public Session login page after OOBE

BUG= 647082 
TEST=Enroll device with Public Session policy enabled.  When Public
Session user pod appears, verify that mouse / touchpad clicks work on
the user pod and shelf buttons.

Review-Url: https://codereview.chromium.org/2361193002
Cr-Commit-Position: refs/heads/master@{#420829}

[modify] https://crrev.com/1031e3dc40e1af209cf973ccaf22ea8b3b409122/ui/login/account_picker/user_pod_row.js

Your change meets the bar and is auto-approved for M54 (branch: 2840)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ddbabb69d4b4f1c3ab0149627ef1e5234ac7d250

commit ddbabb69d4b4f1c3ab0149627ef1e5234ac7d250
Author: glevin <glevin@chromium.org>
Date: Mon Sep 26 14:23:00 2016

Fix clicks on Public Session login page after OOBE

BUG= 647082 
TEST=Enroll device with Public Session policy enabled.  When Public
Session user pod appears, verify that mouse / touchpad clicks work on
the user pod and shelf buttons.

Review-Url: https://codereview.chromium.org/2361193002
Cr-Commit-Position: refs/heads/master@{#420829}
(cherry picked from commit 1031e3dc40e1af209cf973ccaf22ea8b3b409122)

Review URL: https://codereview.chromium.org/2369923002 .

Cr-Commit-Position: refs/branch-heads/2840@{#525}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/ddbabb69d4b4f1c3ab0149627ef1e5234ac7d250/ui/login/account_picker/user_pod_row.js

Verified in Peppy.Both touch and mouse clicks works now.

M	ChromeOS	Chrome	ARC	Type	Channel
54	8743.43.0	54.0.2840.42	3306881	release	beta

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ddbabb69d4b4f1c3ab0149627ef1e5234ac7d250

commit ddbabb69d4b4f1c3ab0149627ef1e5234ac7d250
Author: glevin <glevin@chromium.org>
Date: Mon Sep 26 14:23:00 2016

Fix clicks on Public Session login page after OOBE

BUG= 647082 
TEST=Enroll device with Public Session policy enabled.  When Public
Session user pod appears, verify that mouse / touchpad clicks work on
the user pod and shelf buttons.

Review-Url: https://codereview.chromium.org/2361193002
Cr-Commit-Position: refs/heads/master@{#420829}
(cherry picked from commit 1031e3dc40e1af209cf973ccaf22ea8b3b409122)

Review URL: https://codereview.chromium.org/2369923002 .

Cr-Commit-Position: refs/branch-heads/2840@{#525}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/ddbabb69d4b4f1c3ab0149627ef1e5234ac7d250/ui/login/account_picker/user_pod_row.js