Hi Oshima, based on our conversation. I some thoughts for now. Seems we needs at least two functions that allow us only set display rotation and only update window size:

1. We call display_manager_->SetDisplayRotation() to set the orientation. This will trigger SetRootWindowTransformer() and then UpdateWindowSize().

* We do not want to resize window at this point. We need to have a mechanism to tell SetRootWindowTransformer() that "do not resize now".

2. After SetDisplayRotation(), every status is rotated but not the window size. When rotation animation finish or abort, we need to explicitly UpdateWindowSize(). 

For short term, let's use the old rotation when switching between tablet <-> clamshell.

I am thinking to add a new argument to DisplayConfigurationController::SetDisplayRotation [1] so that in the ScreenRotationAnimator, we know to use the old rotation.

Or we can set a scoped global flag in shell or DisplayConfigurationController, which can be access in ScreenRotationAnimator.

https://cs.chromium.org/chromium/src/ash/display/display_configuration_controller.cc?l=110&rcl=fc5049622fedc52ab603377dd945032faa72f996

I'm fine with either way.

This bug requires manual review: We are only 7 days from stable.
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), ketakid@(ChromeOS), govind@(Desktop)

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ee77a567d98db38bfe3aa72ed4808f4b67beffba

commit ee77a567d98db38bfe3aa72ed4808f4b67beffba
Author: wutao <wutao@chromium.org>
Date: Mon Aug 28 19:11:30 2017

CrOS: Using slow screen rotation animation when existing tablet mode.

Currently screen rotation takes place after a screenshot is taken, which
can cause race condition due to inconsistent state. We plan to modify
the logic to rotate the screen first and then do the animation. In a
short term solution, we force to use slow screen rotation animation when
existing the tablet mode.

Changes:
1. add a new enum RotationAnimation.
2. call with the new enum for rotation when tablet mode is ending.
3. screen_rotation_animator checks the rotation request before rotating.

Bug: 757851
Test: on linux build can call slow screen rotation.
Change-Id: Ia03d14c630600c0be5bd6eb43909759bb6a5e278
Reviewed-on: https://chromium-review.googlesource.com/627270
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497827}
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/display/display_configuration_controller.cc
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/display/display_configuration_controller.h
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/display/display_configuration_controller_unittest.cc
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/display/screen_orientation_controller_chromeos.cc
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/display/screen_orientation_controller_chromeos.h
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/rotator/screen_rotation_animator.cc
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/rotator/screen_rotation_animator.h
[modify] https://crrev.com/ee77a567d98db38bfe3aa72ed4808f4b67beffba/ash/rotator/screen_rotation_animator_unittest.cc

Approving merge to M61.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e

commit 7f636fd4b7c1bb713dbbc03d90f4904a45213c4e
Author: Mitsuru Oshima <oshima@chromium.org>
Date: Wed Aug 30 00:58:12 2017

CrOS: Using slow screen rotation animation when existing tablet mode.

Currently screen rotation takes place after a screenshot is taken, which
can cause race condition due to inconsistent state. We plan to modify
the logic to rotate the screen first and then do the animation. In a
short term solution, we force to use slow screen rotation animation when
existing the tablet mode.

Changes:
1. add a new enum RotationAnimation.
2. call with the new enum for rotation when tablet mode is ending.
3. screen_rotation_animator checks the rotation request before rotating.

(cherry picked from commit ee77a567d98db38bfe3aa72ed4808f4b67beffba)

Bug: 757851
Test: on linux build can call slow screen rotation.
Change-Id: Ia9bc7f8249f802036a72ccdcb19758e85f6ccc92
Reviewed-on: https://chromium-review.googlesource.com/627270
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#497827}
Reviewed-on: https://chromium-review.googlesource.com/642528
Cr-Commit-Position: refs/branch-heads/3163@{#986}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/display/display_configuration_controller.cc
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/display/display_configuration_controller.h
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/display/display_configuration_controller_unittest.cc
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/display/screen_orientation_controller_chromeos.cc
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/display/screen_orientation_controller_chromeos.h
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/rotator/screen_rotation_animator.cc
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/rotator/screen_rotation_animator.h
[modify] https://crrev.com/7f636fd4b7c1bb713dbbc03d90f4904a45213c4e/ash/rotator/screen_rotation_animator_unittest.cc

Hi oshima@, do we still need this?