The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/652240f3d4404a949aaa0a9ca04017bd25011958

commit 652240f3d4404a949aaa0a9ca04017bd25011958
Author: glevin <glevin@chromium.org>
Date: Fri Oct 28 01:33:24 2016

Fix login navigation button enabling

BUG= 658014 , 660011 
TEST=Back, Close, and Refresh buttons should always be disabled when a
modal dialog is present, and enabled otherwise.  See bugs for specific
repro instructions.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2452333003
Cr-Commit-Position: refs/heads/master@{#428231}

[modify] https://crrev.com/652240f3d4404a949aaa0a9ca04017bd25011958/chrome/browser/resources/chromeos/login/oobe_screen_oauth_enrollment.js
[modify] https://crrev.com/652240f3d4404a949aaa0a9ca04017bd25011958/chrome/browser/resources/chromeos/login/screen_gaia_signin.js

As verified in M60.0.3112.39:9592.27.0 elm beta, the fix works according to the described behavior -- the Close (X) button on the top right of enrollment dialog is disabled when the modal dialog is present.

Technically, a modal dialog is a window that forces the user to interact with it before they can go back to using the parent application.

However, during the present of the modal dialog triggered by More options > Learn more, other components such as Shut down, Browse as a guest, change input method, Accessibility, change network are still accessible to the user. A true modal dialog should freeze everything else until it is closed.