Especially when dragging it with two fingers or more fingers above keyboard,
it flashs seriously;

I'm watching the video, but I'm not sure what you mean by flashing. Can you be more specific? Maybe the recording frame rate is too slow to pick up the flashing?

The vedio is too slow ,can not pick up the flashing,
 No special steps, when you switch to floating window, you can with two or more fingers to move the floating window ,it will have serious flashing, also repro in Caroline device;
10439 dev-channel 

Still repro in 10465 dev channel caroline

I see it now. The key was using multiple fingers. The keyboard seems to alternate rapidly between finger locations.

Changing title

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9fd3a2321a09188de82bb37a3f0aa241142cf7b4

commit 9fd3a2321a09188de82bb37a3f0aa241142cf7b4
Author: Blake O'Hare <blakeo@chromium.org>
Date: Tue Apr 10 04:44:08 2018

Fix bug where floating keyboard jitters when dragging with multi-touch

When you use multiple fingers to drag the keyboard around, the events
for these get sent to the event filter sequentially. Check for a
pointer ID (which is unique per touch point) and include it in the
active drag descriptor. Ignore non-matching pointer events.

Bug:  802992 
Change-Id: I5a3948719e5c103ee1aa618afc3fe36939567326
Reviewed-on: https://chromium-review.googlesource.com/979739
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549424}
[modify] https://crrev.com/9fd3a2321a09188de82bb37a3f0aa241142cf7b4/ui/keyboard/container_floating_behavior.cc
[modify] https://crrev.com/9fd3a2321a09188de82bb37a3f0aa241142cf7b4/ui/keyboard/drag_descriptor.cc
[modify] https://crrev.com/9fd3a2321a09188de82bb37a3f0aa241142cf7b4/ui/keyboard/drag_descriptor.h

Still repro in
 6.9.1.1.
68.0.3397.0 10586 clapper

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

vtgjlhkhknk

Verified in 71.0.3544.0 11058.0.0 canary-channel bob 
Input tools version :
71.0.20180906.211816345