The magic thresholds are when one of the tab thumbnails further back is almost fully hidden (at least by the dropshadow).  That's why it's only reproducible when there's lots of tabs.

I suspect an optimization to remove a hidden tab from the layer tree is having a bad side effect.

I have a fix up for review at https://codereview.chromium.org/2253423003

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f859be05a8db1f6301119cd9dfd58d7ddd75ee22

commit f859be05a8db1f6301119cd9dfd58d7ddd75ee22
Author: aelias <aelias@chromium.org>
Date: Fri Aug 19 21:29:13 2016

Make TabLayer recycling sticky to tab_id.

TabListSceneLayer is the parent of all currently visible phone tab
switcher CC layers.  It prefers to reuse existing layers instead of
recreating them from scratch.

However, the old logic was vector-based and had no guaranteed
correlation to tab ids, so when a layer in the middle became covered and
invisible, all layers after it were suddenly repurposed to display a
different tab than on the previous frame (and the last layer in the list
was destroyed, not the layer that became invisible).  The thumbnail
subsystems have problems handling this switchover perfectly and
sometimes flicker white or black for one frame, and this is also
inefficient.

This patch replaces the vector of TabLayers with a map sorted by id, so
that when a TabLayer becomes invisible, that one is removed without
disturbing any of the other layers.

BUG= 638068 

Review-Url: https://codereview.chromium.org/2253423003
Cr-Commit-Position: refs/heads/master@{#413244}

[modify] https://crrev.com/f859be05a8db1f6301119cd9dfd58d7ddd75ee22/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
[modify] https://crrev.com/f859be05a8db1f6301119cd9dfd58d7ddd75ee22/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.h

On second thought, requesting merge back to M53 based on the timeline request on http://b/30863103.

I think this change is relatively safe/isolated although not totally risk-free, I'm inclined to bake it in one dev channel release to check for crashers, then pull it back to 53.

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f66182335a8d477501e0657cafc3600ae9896e98

commit f66182335a8d477501e0657cafc3600ae9896e98
Author: Alexandre Elias <aelias@chromium.org>
Date: Tue Aug 23 19:11:32 2016

Make TabLayer recycling sticky to tab_id.

TabListSceneLayer is the parent of all currently visible phone tab
switcher CC layers.  It prefers to reuse existing layers instead of
recreating them from scratch.

However, the old logic was vector-based and had no guaranteed
correlation to tab ids, so when a layer in the middle became covered and
invisible, all layers after it were suddenly repurposed to display a
different tab than on the previous frame (and the last layer in the list
was destroyed, not the layer that became invisible).  The thumbnail
subsystems have problems handling this switchover perfectly and
sometimes flicker white or black for one frame, and this is also
inefficient.

This patch replaces the vector of TabLayers with a map sorted by id, so
that when a TabLayer becomes invisible, that one is removed without
disturbing any of the other layers.

BUG= 638068 

Review-Url: https://codereview.chromium.org/2253423003
Cr-Commit-Position: refs/heads/master@{#413244}
(cherry picked from commit f859be05a8db1f6301119cd9dfd58d7ddd75ee22)

Review URL: https://codereview.chromium.org/2270563004 .

Cr-Commit-Position: refs/branch-heads/2785@{#730}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/f66182335a8d477501e0657cafc3600ae9896e98/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
[modify] https://crrev.com/f66182335a8d477501e0657cafc3600ae9896e98/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f66182335a8d477501e0657cafc3600ae9896e98

commit f66182335a8d477501e0657cafc3600ae9896e98
Author: Alexandre Elias <aelias@chromium.org>
Date: Tue Aug 23 19:11:32 2016

Make TabLayer recycling sticky to tab_id.

TabListSceneLayer is the parent of all currently visible phone tab
switcher CC layers.  It prefers to reuse existing layers instead of
recreating them from scratch.

However, the old logic was vector-based and had no guaranteed
correlation to tab ids, so when a layer in the middle became covered and
invisible, all layers after it were suddenly repurposed to display a
different tab than on the previous frame (and the last layer in the list
was destroyed, not the layer that became invisible).  The thumbnail
subsystems have problems handling this switchover perfectly and
sometimes flicker white or black for one frame, and this is also
inefficient.

This patch replaces the vector of TabLayers with a map sorted by id, so
that when a TabLayer becomes invisible, that one is removed without
disturbing any of the other layers.

BUG= 638068 

Review-Url: https://codereview.chromium.org/2253423003
Cr-Commit-Position: refs/heads/master@{#413244}
(cherry picked from commit f859be05a8db1f6301119cd9dfd58d7ddd75ee22)

Review URL: https://codereview.chromium.org/2270563004 .

Cr-Commit-Position: refs/branch-heads/2785@{#730}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/f66182335a8d477501e0657cafc3600ae9896e98/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
[modify] https://crrev.com/f66182335a8d477501e0657cafc3600ae9896e98/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/28162ee142c9e5008fe16a3feb53734d89522f92

commit 28162ee142c9e5008fe16a3feb53734d89522f92
Author: Alexandre Elias <aelias@chromium.org>
Date: Wed Aug 24 19:09:50 2016

Make TabLayer recycling sticky to tab_id.

TabListSceneLayer is the parent of all currently visible phone tab
switcher CC layers.  It prefers to reuse existing layers instead of
recreating them from scratch.

However, the old logic was vector-based and had no guaranteed
correlation to tab ids, so when a layer in the middle became covered and
invisible, all layers after it were suddenly repurposed to display a
different tab than on the previous frame (and the last layer in the list
was destroyed, not the layer that became invisible).  The thumbnail
subsystems have problems handling this switchover perfectly and
sometimes flicker white or black for one frame, and this is also
inefficient.

This patch replaces the vector of TabLayers with a map sorted by id, so
that when a TabLayer becomes invisible, that one is removed without
disturbing any of the other layers.

[edited to add more #includes: see http://crbug.com/640352]

BUG= 638068 ,640352

Review URL: https://codereview.chromium.org/2274993002 .

Review-Url: https://codereview.chromium.org/2253423003
Cr-Original-Commit-Position: refs/heads/master@{#413244}
Cr-Commit-Position: refs/branch-heads/2785@{#740}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/28162ee142c9e5008fe16a3feb53734d89522f92/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
[modify] https://crrev.com/28162ee142c9e5008fe16a3feb53734d89522f92/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.h

For the record, in between #9 and #11, I reverted the patch on 53 branch as https://codereview.chromium.org/2272603004/ due to the missing #include (another header was papering over the problem on ToT so it was only caught on cherry-pick).  For some reason bugdroid didn't post about the revert.  In any case, now relanded and fixed.

 Issue 502998  has been merged into this issue.