Hi Blakeo&Hanada,
To avoid keyboard flash, I am going to add parameter "bound" for API "chrome.virtualKeyboardPrivate.setMode" to setup initial keyboard container bound. what's your opinion?

As we discuss offline. 
Blake will add size parameter for chrome.virtualKeyboardPrivate.setMode API.

and here is a CL for adding the parameter crrev.com/c/942281

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c972bfd116979ea534a77401fe6da19fdfcf9e5f

commit c972bfd116979ea534a77401fe6da19fdfcf9e5f
Author: Yuichiro Hanada <yhanada@chromium.org>
Date: Sat Mar 17 00:11:10 2018

Add new method, virtualKeyboardPrivate.setContainerBehavior().

virtualKeyboardPrivate.setMode() lacks 'bounds' parameter to set the
virtual keyboard bounds after changing mode.
The new method will replace setMode() with an ability to set bounds of
the virtual keyboard container after changing mode.
It's needed to prevent flashing when changing virtual keyboard
mode. We want to change the window bounds before starting showing
animation.

Bug:  776260 ,  802991 
Test: Manual.
Change-Id: Iacf9977d06e05ac4dd3c66175cae7ba87d5fa0c6
Reviewed-on: https://chromium-review.googlesource.com/942281
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Blake O'Hare <blakeo@chromium.org>
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543878}
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/ash/wm/lock_layout_manager_unittest.cc
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/chrome/browser/extensions/api/virtual_keyboard_private/chrome_virtual_keyboard_delegate.cc
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/chrome/browser/extensions/api/virtual_keyboard_private/chrome_virtual_keyboard_delegate.h
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/browser/api/virtual_keyboard_private/virtual_keyboard_delegate.h
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/browser/api/virtual_keyboard_private/virtual_keyboard_private_api.cc
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/browser/api/virtual_keyboard_private/virtual_keyboard_private_api.h
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/browser/extension_function_histogram_value.h
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/common/api/virtual_keyboard_private.json
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/shell/browser/shell_virtual_keyboard_delegate.cc
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/extensions/shell/browser/shell_virtual_keyboard_delegate.h
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/ui/keyboard/keyboard_controller.cc
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/ui/keyboard/keyboard_controller.h
[modify] https://crrev.com/c972bfd116979ea534a77401fe6da19fdfcf9e5f/ui/keyboard/keyboard_controller_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/53e7cfc125c588b1ea72a6be6141d1a64be30e09

commit 53e7cfc125c588b1ea72a6be6141d1a64be30e09
Author: Yuichiro Hanada <yhanada@chromium.org>
Date: Fri Mar 23 00:23:56 2018

Use the passed target bounds from IME to preset size of VK window.

This CL makes KeyboardController use the passed target bounds of VK
window after changing a container behavior mode. The VK window will be
resized to the passed size before showing animation starts to prevent
the VK window flashing.

Bug:  802991 
Test: Unit test + manual testing.
Change-Id: I0904fd47a0a217de4eaf5afeaceec0a7e1e1dd52
Reviewed-on: https://chromium-review.googlesource.com/970382
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Blake O'Hare <blakeo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545304}
[modify] https://crrev.com/53e7cfc125c588b1ea72a6be6141d1a64be30e09/ui/keyboard/keyboard_controller.cc
[modify] https://crrev.com/53e7cfc125c588b1ea72a6be6141d1a64be30e09/ui/keyboard/keyboard_controller_unittest.cc
[modify] https://crrev.com/53e7cfc125c588b1ea72a6be6141d1a64be30e09/ui/keyboard/queued_container_type.cc
[modify] https://crrev.com/53e7cfc125c588b1ea72a6be6141d1a64be30e09/ui/keyboard/queued_container_type.h

blakeo@ is no longer working on virtual keyboard. Marking as available.

Not repro. It should be fixed now. Close it.

Verified on the latest version: 70.0.3544.0, 11057.0.0 canary-channel kevin.