Does this require a quick open, or does it also show up sometimes if you close the lid, wait for the system to suspend (e.g. at least 5-10 seconds), and then reopen the lid?

I'm still not sure whether Chrome delays reporting suspend readiness until the window-hide animation has finished or not. I agree that we should skip the hiding animation on lid-closed in any case, though.

Under my test, it must be a quick reopen. In the email thread, the reporter did say the repro step with "(3) wait". But I don't think lock animations will be blocked for any reason. So if waiting 5~10s, lock screen must be ready.

From the code, chrome reports suspend readiness after lock screen animations are done. Probably they are not needed once lock screen animations are immediate per  issue 748732 .

Perhaps, but I still have some concerns regarding making sure that we're waiting long enough for the compositor (which I believe is working asynchronously here) to finish drawing the frame with the windows hidden before we suspend. I don't know if there's anything that guarantees that this will happen.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a353d28ce10401b486139b79db6206757b94d3e9

commit a353d28ce10401b486139b79db6206757b94d3e9
Author: Qiang Xu <warx@chromium.org>
Date: Fri Aug 04 01:05:49 2017

cros: hide non lock screen containers immediately for suspend imminent induced
locking screen

changes:
Make hiding non lock screen containers immediately for suspend imminent
induced locking screen. This will prevent the possibility of peeking the
glimpse of unlocked screen if we close/reopen lid quickly.

Also added test coverage:
--gtest_filter=PowerEventObserverTest.NonLockScreenContainersHideAnimation

Test: tested on device, bug is fixed.
Bug:  751908 
Change-Id: I39ab6f8b69c1c7632c4e1d4d502b704ea61f6aa6
Reviewed-on: https://chromium-review.googlesource.com/601090
Commit-Queue: Qiang(Joe) Xu <warx@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491900}
[modify] https://crrev.com/a353d28ce10401b486139b79db6206757b94d3e9/ash/system/power/power_event_observer.cc
[modify] https://crrev.com/a353d28ce10401b486139b79db6206757b94d3e9/ash/system/power/power_event_observer_unittest.cc

This bug requires manual review: M61 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), ketakid@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Approving merge to M61 Chrome OS.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b3874b5c910e6126f6027c9fb2db06b7f96d965e

commit b3874b5c910e6126f6027c9fb2db06b7f96d965e
Author: Qiang Xu <warx@chromium.org>
Date: Tue Aug 08 17:44:58 2017

[merge to m61] cros: hide non lock screen containers immediately for suspend imminent induced locking screen

changes:
Make hiding non lock screen containers immediately for suspend imminent
induced locking screen. This will prevent the possibility of peeking the
glimpse of unlocked screen if we close/reopen lid quickly.

Also added test coverage:
--gtest_filter=PowerEventObserverTest.NonLockScreenContainersHideAnimation

TBR: derat@chromium.org

(cherry picked from commit a353d28ce10401b486139b79db6206757b94d3e9)

Test: tested on device, bug is fixed.
Bug:  751908 
Change-Id: I39ab6f8b69c1c7632c4e1d4d502b704ea61f6aa6
Reviewed-on: https://chromium-review.googlesource.com/601090
Commit-Queue: Qiang(Joe) Xu <warx@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#491900}
Reviewed-on: https://chromium-review.googlesource.com/606787
Reviewed-by: Qiang(Joe) Xu <warx@chromium.org>
Cr-Commit-Position: refs/branch-heads/3163@{#383}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
[modify] https://crrev.com/b3874b5c910e6126f6027c9fb2db06b7f96d965e/ash/system/power/power_event_observer.cc
[modify] https://crrev.com/b3874b5c910e6126f6027c9fb2db06b7f96d965e/ash/system/power/power_event_observer_unittest.cc