You can see what it currently looks like (in slow-motion) at https://drive.google.com/file/d/19-ygX5rxiut-GfJluKGsG1q84W1QgHWI/view?usp=sharing .

I'd recommend restarting the reveal animation from whatever point it is currently at. That should address the flashing jank. As derat@ says, I wouldn't use Pixel 2 as a model.

+1, agree with Dan and Ben

I don't quite get the meaning here, I think what Dan and Ben suggested are different.
Dan suggested to start the pre-shutdown fade-to-white animation if press the power button when menu is shown.
Ben suggested to restart the showing menu animation in this case. I think this is what we have now.

Do I understand correct? Which one it should be?

I think both Dan's and my suggestions apply, just at different moments. Dan's suggestion sounds like his comments in crbug.com/826064 #5.

I was referring to the jank I saw pressing the button again when the menu was partially showing - with the animation restarting from the beginning. I think Dan's comment applies when the menu is already fully visible.

Yes, I agree with the summary in #7. :-)

I see. Thanks.

Dan, we can start the pre-shutdown animation if press the power button when menu is opened and focus is on the "power-off" item. I think we may don't start the pre-shutdown animation if the focus is on "sign-out" item? Then what should we do in this case?

How about request focus for "power-off" item and then start the pre-shtudown animation?

I think we should always start the pre-shutdown animation if the power button is held while the menu is open, regardless of where the focus is.

Focusing the "power off" item when starting the pre-shutdown animation to make it clearer what's going to happen sounds like a good idea to me.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e

commit b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e
Author: Min Chen <minch@google.com>
Date: Fri Apr 13 23:11:32 2018

ash: Start pre-shutdown animation if long press power button
when menu is already shown.

- Don't dismiss the menu if pressing power button when menu is already shown.
  Wait for 650ms to start the pre-shutdown animation and request focus for
  'power off' item instead.

- Turn screen off for convertible device / in tablet mode if
  "ModeSpecificPowerButton" is set when menu is not fully shown or second
   press before starting the pre-shutdown animation.

Test=PowerButtonControllerTest*

Bug:  826057 
Change-Id: I1f906e2bcf5d91960180742bf558c00784fbaa1e
Reviewed-on: https://chromium-review.googlesource.com/1011590
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Qiang Xu <warx@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550785}
[modify] https://crrev.com/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e/ash/system/power/power_button_controller.cc
[modify] https://crrev.com/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e/ash/system/power/power_button_controller.h
[modify] https://crrev.com/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e/ash/system/power/power_button_controller_unittest.cc

Pressing the power button for 650ms when menu is opened will start the pre-shutdown animation.

Your change meets the bar and is auto-approved for M67. Please go ahead and merge the CL to branch 3396 manually. Please contact milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), govind@(Desktop)

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

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e

commit b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e
Author: Min Chen <minch@google.com>
Date: Fri Apr 13 23:11:32 2018

ash: Start pre-shutdown animation if long press power button
when menu is already shown.

- Don't dismiss the menu if pressing power button when menu is already shown.
  Wait for 650ms to start the pre-shutdown animation and request focus for
  'power off' item instead.

- Turn screen off for convertible device / in tablet mode if
  "ModeSpecificPowerButton" is set when menu is not fully shown or second
   press before starting the pre-shutdown animation.

Test=PowerButtonControllerTest*

Bug:  826057 
Change-Id: I1f906e2bcf5d91960180742bf558c00784fbaa1e
Reviewed-on: https://chromium-review.googlesource.com/1011590
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Qiang Xu <warx@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550785}
[modify] https://crrev.com/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e/ash/system/power/power_button_controller.cc
[modify] https://crrev.com/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e/ash/system/power/power_button_controller.h
[modify] https://crrev.com/b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e/ash/system/power/power_button_controller_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e801faea3a9e294da4fd761955f4618da3a43238

commit e801faea3a9e294da4fd761955f4618da3a43238
Author: Min Chen <minch@google.com>
Date: Tue Apr 17 18:11:53 2018

[Merge to M67]ash: Start pre-shutdown animation if long press power button when menu is already shown.

TBR=minch@chromium.org

- Don't dismiss the menu if pressing power button when menu is already shown.
  Wait for 650ms to start the pre-shutdown animation and request focus for
  'power off' item instead.

- Turn screen off for convertible device / in tablet mode if
  "ModeSpecificPowerButton" is set when menu is not fully shown or second
   press before starting the pre-shutdown animation.

Test=PowerButtonControllerTest*

(cherry picked from commit b432b7c1ef34b32400fbc2d6f9bfa4624e4ea32e)

Bug:  826057 
Change-Id: I1f906e2bcf5d91960180742bf558c00784fbaa1e
Reviewed-on: https://chromium-review.googlesource.com/1011590
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Qiang Xu <warx@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#550785}
Reviewed-on: https://chromium-review.googlesource.com/1015524
Reviewed-by: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/branch-heads/3396@{#52}
Cr-Branched-From: 9ef2aa869bc7bc0c089e255d698cca6e47d6b038-refs/heads/master@{#550428}
[modify] https://crrev.com/e801faea3a9e294da4fd761955f4618da3a43238/ash/system/power/power_button_controller.cc
[modify] https://crrev.com/e801faea3a9e294da4fd761955f4618da3a43238/ash/system/power/power_button_controller.h
[modify] https://crrev.com/e801faea3a9e294da4fd761955f4618da3a43238/ash/system/power/power_button_controller_unittest.cc

Verified on M67 (10575.26.0, 67.0.3396.31)