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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6d7e87248179c7c06f5d06a9ccf46c9964d9fd98

commit 6d7e87248179c7c06f5d06a9ccf46c9964d9fd98
Author: Mark Cogan <marq@google.com>
Date: Thu Apr 19 12:36:16 2018

[iOS] Trigger correct animation for new Tab grid cells.

This CL tweaks how visible cells are detected so that the grid zoom
animation is triggered when a new tab is created.

The TabGridTransitionHandler chooses which animation to use for BVC
presentation by checking to see if the selected grid cell is visible.

This was handled by checking the collection view's selected cells.
However, when a new cell is added, the selected cell in the collection
view isn't updated until the completion block of the batchUpdates: call.

The view controller presentation is started synchronously with the
batchUpdates: invocation, so the visibility of the selected cell is
determined before the completion block runs.

The change this CL makes is to use the internal -selectedItemIndex to
determine if the selected cell is visible. This is updated as part of
the collection view updates, so it is accurate before the completion runs.

Bug:  804586 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I54fe213847d1a7da8b0fcd019a848885f8dfe9ba
Reviewed-on: https://chromium-review.googlesource.com/1016911
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551992}
[modify] https://crrev.com/6d7e87248179c7c06f5d06a9ccf46c9964d9fd98/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.mm

Issue 834978 has been merged into this issue.

The NextAction date has arrived: 2018-04-25