After thinking about it more, fixing this bug is going to require a mix of things. The white background and animation should still be removed outright for Chrome windows (and probably ARC app windows too, but I don't know how they draw their top bars under the hood). Web / native apps likely need to retain their top bar backgrounds during the transition, because for those kinds of windows it looks like the actual top bars do the animating.

Please see this section of the window management motion spec for the spec and close-up slowmo videos showing the transition we want for the top bars: https://mccanny.users.x20web.corp.google.com/www/wm-motion/index.html#overview-freeform

The transition should look like this for all apps except Chrome, which shouldn't have any special animation because it keeps its top chrome within the thumbnail.

If there are performance issues with animating the window mask we can explore workarounds, just let me know.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/96cd65fbfb09cfd5dd6e1ed572373071e186b291

commit 96cd65fbfb09cfd5dd6e1ed572373071e186b291
Author: Sammie Quon <sammiequon@google.com>
Date: Thu Mar 01 03:42:44 2018

overview: Do not animate in header of tablet mode maximized windows.

Their headers are hidden already, so do not need to animate them in when
entering overview.

Test: manual
Bug:  812004 
Change-Id: If3528f2e4296d626ca817bb826dac2473d32fa5d
Reviewed-on: https://chromium-review.googlesource.com/941781
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540022}
[modify] https://crrev.com/96cd65fbfb09cfd5dd6e1ed572373071e186b291/ash/wm/overview/window_selector_item.cc

"overview: Do not animate in header of tablet mode maximized windows"

Could that be removed from clamshell mode as well? The animated headers just look too "busy"

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/07571b54ec6bbc30f287259506e0c49005dde5a6

commit 07571b54ec6bbc30f287259506e0c49005dde5a6
Author: Sammie Quon <sammiequon@google.com>
Date: Wed Mar 14 19:07:32 2018

overview: Do not animate headers on enter/exit for tablet mode.

Spec : https://mccanny.users.x20web.corp.google.com/www/wm-motion/index.html#overview-freeform
We do not want to move the headers when entering tablet mode, they
should already be at their final destination.

Test: manual
Bug:  812004 
Change-Id: Id5ab7e7e4f56da7ecf4c83f76ffa01aa49d6e661
Reviewed-on: https://chromium-review.googlesource.com/958223
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543149}
[modify] https://crrev.com/07571b54ec6bbc30f287259506e0c49005dde5a6/ash/wm/overview/overview_animation_type.h
[modify] https://crrev.com/07571b54ec6bbc30f287259506e0c49005dde5a6/ash/wm/overview/scoped_overview_animation_settings.cc
[modify] https://crrev.com/07571b54ec6bbc30f287259506e0c49005dde5a6/ash/wm/overview/window_selector_item.cc
[modify] https://crrev.com/07571b54ec6bbc30f287259506e0c49005dde5a6/ash/wm/overview/window_selector_item.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fc2d77d50dcc5d418ad1975b9a07826b16d8708a

commit fc2d77d50dcc5d418ad1975b9a07826b16d8708a
Author: Sammie Quon <sammiequon@google.com>
Date: Wed Apr 11 05:43:33 2018

overivew: Animate blur when there are no maximized windows.

See bug(s) for spec. This CL adds an animated blur on wallpaper when entering/existing
overview when no maximized/fullscreen windows.

Bug: 823533,  812004 
Change-Id: Icf8bdababedaaefe869f62768aacf7abcb3643d2
Reviewed-on: https://chromium-review.googlesource.com/979167
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549780}
[modify] https://crrev.com/fc2d77d50dcc5d418ad1975b9a07826b16d8708a/ash/wm/overview/window_grid.cc
[modify] https://crrev.com/fc2d77d50dcc5d418ad1975b9a07826b16d8708a/ash/wm/overview/window_selector.cc
[modify] https://crrev.com/fc2d77d50dcc5d418ad1975b9a07826b16d8708a/ash/wm/overview/window_selector.h
[modify] https://crrev.com/fc2d77d50dcc5d418ad1975b9a07826b16d8708a/ash/wm/overview/window_selector_controller.cc
[modify] https://crrev.com/fc2d77d50dcc5d418ad1975b9a07826b16d8708a/ash/wm/overview/window_selector_controller.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bdcec7ae1dab8d49e5f3991307f865316004088a

commit bdcec7ae1dab8d49e5f3991307f865316004088a
Author: Findit <findit-for-me@appspot.gserviceaccount.com>
Date: Wed Apr 11 11:10:19 2018

Revert "overivew: Animate blur when there are no maximized windows."

This reverts commit fc2d77d50dcc5d418ad1975b9a07826b16d8708a.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 549780 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtL2ZjMmQ3N2Q1MGRjYzVkNDE4YWQxOTc1YjlhMDc4MjZiMTZkODcwOGEM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-dbg/5150

Sample Failed Step: ash_unittests

Original change's description:
> overivew: Animate blur when there are no maximized windows.
> 
> See bug(s) for spec. This CL adds an animated blur on wallpaper when entering/existing
> overview when no maximized/fullscreen windows.
> 
> Bug: 823533,  812004 
> Change-Id: Icf8bdababedaaefe869f62768aacf7abcb3643d2
> Reviewed-on: https://chromium-review.googlesource.com/979167
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#549780}

Change-Id: If1226fa286e8f82885cea814f5fb89e0253ef98a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 823533,  812004 
Reviewed-on: https://chromium-review.googlesource.com/1006894
Cr-Commit-Position: refs/heads/master@{#549855}
[modify] https://crrev.com/bdcec7ae1dab8d49e5f3991307f865316004088a/ash/wm/overview/window_grid.cc
[modify] https://crrev.com/bdcec7ae1dab8d49e5f3991307f865316004088a/ash/wm/overview/window_selector.cc
[modify] https://crrev.com/bdcec7ae1dab8d49e5f3991307f865316004088a/ash/wm/overview/window_selector.h
[modify] https://crrev.com/bdcec7ae1dab8d49e5f3991307f865316004088a/ash/wm/overview/window_selector_controller.cc
[modify] https://crrev.com/bdcec7ae1dab8d49e5f3991307f865316004088a/ash/wm/overview/window_selector_controller.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f

commit 8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f
Author: Sammie Quon <sammiequon@google.com>
Date: Fri Apr 13 00:02:47 2018

Reland "overivew: Animate blur when there are no maximized windows."

This is a reland of fc2d77d50dcc5d418ad1975b9a07826b16d8708a

TBR=oshima@chromium.org

Original change's description:
> overivew: Animate blur when there are no maximized windows.
>
> See bug(s) for spec. This CL adds an animated blur on wallpaper when entering/existing
> overview when no maximized/fullscreen windows.
>
> Bug: 823533,  812004 
> Change-Id: Icf8bdababedaaefe869f62768aacf7abcb3643d2
> Reviewed-on: https://chromium-review.googlesource.com/979167
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#549780}

Bug: 823533,  812004 
Change-Id: I3aa8ded00f07209108f144bddfa401a35d90ff2d
Reviewed-on: https://chromium-review.googlesource.com/1007742
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550422}
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_grid.cc
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector.cc
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector.h
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector_controller.cc
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector_controller.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f

commit 8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f
Author: Sammie Quon <sammiequon@google.com>
Date: Fri Apr 13 00:02:47 2018

Reland "overivew: Animate blur when there are no maximized windows."

This is a reland of fc2d77d50dcc5d418ad1975b9a07826b16d8708a

TBR=oshima@chromium.org

Original change's description:
> overivew: Animate blur when there are no maximized windows.
>
> See bug(s) for spec. This CL adds an animated blur on wallpaper when entering/existing
> overview when no maximized/fullscreen windows.
>
> Bug: 823533,  812004 
> Change-Id: Icf8bdababedaaefe869f62768aacf7abcb3643d2
> Reviewed-on: https://chromium-review.googlesource.com/979167
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#549780}

Bug: 823533,  812004 
Change-Id: I3aa8ded00f07209108f144bddfa401a35d90ff2d
Reviewed-on: https://chromium-review.googlesource.com/1007742
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550422}
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_grid.cc
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector.cc
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector.h
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector_controller.cc
[modify] https://crrev.com/8ba3f1b68c2ebde3b6336bfb4f9ebd294608b37f/ash/wm/overview/window_selector_controller.h