The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/01191ed1f32cbccb798f6df1d7e67ea6c5aa5cdf

commit 01191ed1f32cbccb798f6df1d7e67ea6c5aa5cdf
Author: Mark Cogan <marq@google.com>
Date: Wed Aug 01 19:12:52 2018

[iOS] Fix white flicker in iPad tab grid transitions.

Exiting the tab grid on iPad, especially for tabs close to the top of the screen, shows a white flicker in the toolbar area of the tab.

This is due to the fade in/fade out of the grid cell header and the toolbar snapshot, which transitions through a state where the header is totally white. The timing is such that this can happen right when the top of the tab is in position, making for a distinct flash in the toolbar.

This CL fixes this by using a crossfade instead of the fade in/ fade out. The transition is fast enough that the crossfade doesn't look muddy, and it remove the abrupt flash.

Bug: 865440
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ifbceef6a79cbde4ea08c76b6f7e6a35bd8225330
Reviewed-on: https://chromium-review.googlesource.com/1158832
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579888}
[modify] https://crrev.com/01191ed1f32cbccb798f6df1d7e67ea6c5aa5cdf/ios/chrome/browser/ui/tab_grid/transitions/grid_transition_animation.mm

Verified on Canary.

Approved.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c25402f83ef9f6135e571d485e463e061e0932de

commit c25402f83ef9f6135e571d485e463e061e0932de
Author: Mark Cogan <marq@google.com>
Date: Fri Aug 03 06:23:35 2018

[iOS] Fix white flicker in iPad tab grid transitions.

Exiting the tab grid on iPad, especially for tabs close to the top of the screen, shows a white flicker in the toolbar area of the tab.

This is due to the fade in/fade out of the grid cell header and the toolbar snapshot, which transitions through a state where the header is totally white. The timing is such that this can happen right when the top of the tab is in position, making for a distinct flash in the toolbar.

This CL fixes this by using a crossfade instead of the fade in/ fade out. The transition is fast enough that the crossfade doesn't look muddy, and it remove the abrupt flash.

Bug: 865440
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ifbceef6a79cbde4ea08c76b6f7e6a35bd8225330
Reviewed-on: https://chromium-review.googlesource.com/1158832
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#579888}(cherry picked from commit 01191ed1f32cbccb798f6df1d7e67ea6c5aa5cdf)
Reviewed-on: https://chromium-review.googlesource.com/1161063
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/branch-heads/3497@{#375}
Cr-Branched-From: 271eaf50594eb818c9295dc78d364aea18c82ea8-refs/heads/master@{#576753}
[modify] https://crrev.com/c25402f83ef9f6135e571d485e463e061e0932de/ios/chrome/browser/ui/tab_grid/transitions/grid_transition_animation.mm

I can still see a white hair line at the top of the toolbar.
Tested on M70.0.3515.0 canary, iPad Pro 10.5" iOS12.0 beta#6.

https://drive.google.com/file/d/1k8Zt7NXULuomkQtz0wNZ4utK3j58zXV_/view

From the recorded video I don't see anything, but on the device when I perform tab grid animation (entry and exit), I can clearly see a white line at top of the screen.

I looked at this on an iPad, and while there is a slight white flash when opening a tab towards the top of the grid, I think it's acceptable to ship in M69.

I'll keep this bug open for further tweaks in M70.

I looked this over again, and it's not a P1.