Repro steps:
- change shelf alignment to left or right
- toggle tablet mode

 Issue 813433  has been merged into this issue.

Which versions does this happen on?

I feel this is not regression. The DUP indicates it also happens on m65.

Yeah, i would guess it came up when i refactored the back button, which is m65.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b3a893d388a5f001fc83132aa7342552b9660f54

commit b3a893d388a5f001fc83132aa7342552b9660f54
Author: Sammie Quon <sammiequon@google.com>
Date: Thu Apr 12 16:06:11 2018

shelf: Back button shows up when entering tablet with left shelf.

Back button visibility was normally handled on shelf animation finish.
When switching from vertical shelf to tablet mode (force bottom shelf),
there is no animation, so the back button does not show up. This cl
handles that case.

Test: ash_unittests BackButtonTest.*
Bug:  831469 
Change-Id: Ia95e791d30deb5a5c91599039c39094ffa29003b
Reviewed-on: https://chromium-review.googlesource.com/1008163
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550223}
[modify] https://crrev.com/b3a893d388a5f001fc83132aa7342552b9660f54/ash/shelf/back_button_unittest.cc
[modify] https://crrev.com/b3a893d388a5f001fc83132aa7342552b9660f54/ash/shelf/shelf_view.cc
[modify] https://crrev.com/b3a893d388a5f001fc83132aa7342552b9660f54/ash/shelf/shelf_view.h

This bug requires manual review: We are only 4 days from stable.
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), josafat@(ChromeOS), abdulsyed@(Desktop)

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

+josafat for permissions

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fced5ca44d731093920d059dbfc67fc64d97baec

commit fced5ca44d731093920d059dbfc67fc64d97baec
Author: Sammie Quon <sammiequon@google.com>
Date: Fri Apr 13 16:51:16 2018

[merge to m66] shelf: Back button shows up when entering tablet with left shelf.

Back button visibility was normally handled on shelf animation finish.
When switching from vertical shelf to tablet mode (force bottom shelf),
there is no animation, so the back button does not show up. This cl
handles that case.

TBR=sammiequon@google.com

(cherry picked from commit b3a893d388a5f001fc83132aa7342552b9660f54)

Test: ash_unittests BackButtonTest.*
Bug:  831469 
Change-Id: Ia95e791d30deb5a5c91599039c39094ffa29003b
Reviewed-on: https://chromium-review.googlesource.com/1008163
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#550223}
Reviewed-on: https://chromium-review.googlesource.com/1012189
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/branch-heads/3359@{#702}
Cr-Branched-From: 66afc5e5d10127546cc4b98b9117aff588b5e66b-refs/heads/master@{#540276}
[modify] https://crrev.com/fced5ca44d731093920d059dbfc67fc64d97baec/ash/shelf/back_button_unittest.cc
[modify] https://crrev.com/fced5ca44d731093920d059dbfc67fc64d97baec/ash/shelf/shelf_view.cc
[modify] https://crrev.com/fced5ca44d731093920d059dbfc67fc64d97baec/ash/shelf/shelf_view.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b3a893d388a5f001fc83132aa7342552b9660f54

commit b3a893d388a5f001fc83132aa7342552b9660f54
Author: Sammie Quon <sammiequon@google.com>
Date: Thu Apr 12 16:06:11 2018

shelf: Back button shows up when entering tablet with left shelf.

Back button visibility was normally handled on shelf animation finish.
When switching from vertical shelf to tablet mode (force bottom shelf),
there is no animation, so the back button does not show up. This cl
handles that case.

Test: ash_unittests BackButtonTest.*
Bug:  831469 
Change-Id: Ia95e791d30deb5a5c91599039c39094ffa29003b
Reviewed-on: https://chromium-review.googlesource.com/1008163
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550223}
[modify] https://crrev.com/b3a893d388a5f001fc83132aa7342552b9660f54/ash/shelf/back_button_unittest.cc
[modify] https://crrev.com/b3a893d388a5f001fc83132aa7342552b9660f54/ash/shelf/shelf_view.cc
[modify] https://crrev.com/b3a893d388a5f001fc83132aa7342552b9660f54/ash/shelf/shelf_view.h