There is the code that draws the frame
https://cs.chromium.org/chromium/src/ash/frame/default_frame_header.cc?rcl=8dd1342810136382baa61dff6e1a8711c51d5971&l=52

and this shadow_bounds has the size in pixel
https://cs.chromium.org/chromium/src/components/exo/shell_surface_base.h?rcl=4490a0547e271f1d765c65d587d0eca6baba508a&l=310

what's the status of this?

no much progress, I'm working on other stuffs.

ok, I'll take this. I have CL and hopefully can send soon.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/29b93df10d43e61cbccd6b7330dce05e40f848a0

commit 29b93df10d43e61cbccd6b7330dce05e40f848a0
Author: Mitsuru Oshima <oshima@chromium.org>
Date: Fri May 25 01:18:48 2018

Use pixel width to paint the frame background for ARC++

ARC's widget bounds and actual arc task bounds aren't exactly the same.
Use the shadow bounds, which is given in pixels, to draw the frame's
background to match the arc task width.

BUG:  836331 
TEST: manually tested with dsf 1,2, 2.25 with arc apps

Change-Id: Id19316aa9f52ca07bb44d24ef22fc7ca78da2a6f
Reviewed-on: https://chromium-review.googlesource.com/1071487
Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561723}
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/ash/frame/default_frame_header.cc
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/ash/frame/default_frame_header.h
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/ash/frame/frame_header.cc
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/ash/frame/frame_header.h
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/ash/frame/header_view.cc
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/ash/frame/header_view.h
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/components/exo/client_controlled_shell_surface.cc
[modify] https://crrev.com/29b93df10d43e61cbccd6b7330dce05e40f848a0/components/exo/client_controlled_shell_surface.h