This isn't exactly a regression, it's always been present on O and we somehow haven't noticed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7363cdb51f222c9a9afc1f3004d412c371736486

commit 7363cdb51f222c9a9afc1f3004d412c371736486
Author: Michael Thiessen <mthiesse@chromium.org>
Date: Thu Feb 08 17:33:08 2018

VR: Fix 2D UI flash on headset insertion entering VR.

Setting the window mode too early leads to it racily being ignored by
the platform, and the 150ms we allowed for window animations wasn't
enough in some cases, especially in debug builds.

Bug:  810039 
Change-Id: I73873ce5ee99ea0ab21cfeb4d53f89b7bd97f979
Reviewed-on: https://chromium-review.googlesource.com/908719
Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535425}
[modify] https://crrev.com/7363cdb51f222c9a9afc1f3004d412c371736486/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellDelegate.java

Will request merge after this has spend some time baking in canary and we make sure it doesn't regress Deep-linked VR apps.

 Issue 810444  has been merged into this issue.

This bug requires manual review: M65 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), bhthompson@(ChromeOS), govind@(Desktop)

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

updating ownership to reflect reality

Is this fix stable enough for M65? It looks simple but could it get worse if some conditions are not met? Since this issue is not a regression, can we let the fix bake and ship it in M66 instead?

In the worst case, this fix might not always fix the issue, but it shouldn't ever make it worse (and any risk associated with this CL is limited to VR users).

I would prefer to merge this back to 65, as the bright 2D UI flash is really jarring for VR users.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/34a0ccba442b31d3e6eefc3c43d3eba63a8fe3bf

commit 34a0ccba442b31d3e6eefc3c43d3eba63a8fe3bf
Author: Michael Thiessen <mthiesse@chromium.org>
Date: Wed Feb 14 20:42:01 2018

VR: Fix 2D UI flash on headset insertion entering VR.

Setting the window mode too early leads to it racily being ignored by
the platform, and the 150ms we allowed for window animations wasn't
enough in some cases, especially in debug builds.

TBR=mthiesse@chromium.org

(cherry picked from commit 7363cdb51f222c9a9afc1f3004d412c371736486)

Bug:  810039 
Change-Id: I73873ce5ee99ea0ab21cfeb4d53f89b7bd97f979
Reviewed-on: https://chromium-review.googlesource.com/908719
Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#535425}
Reviewed-on: https://chromium-review.googlesource.com/919317
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#468}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
[modify] https://crrev.com/34a0ccba442b31d3e6eefc3c43d3eba63a8fe3bf/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellDelegate.java

This is covered by several manual tests.