Observation: before opening the CRD window, when the mouse is over the skyline, the events are dispatched to an unnamed window, which is highly likely to be the skyline. After opening the CRD window, mouse events in this area are dispatched to RootWindow-0.

More curious, before opening the CRD window, when the mouse is over the area to the right of the window, it's dispatched to the WallpaperView, properly. However, after opening the CRD window, this area is also dispatched to RootWindow-0!

It's *not* the whole height of the skyline. There is ~3 pixel high area on the bottom that is still accessible.

Follow up experiment: 

1) Start chrome --mus and move the window down to the middle of the screen.

2) Start CRD.

3) Note that mouse events are being dispatched correctly to the skyline.

4) Drag the browser window back to the top of the screen.

5) Note that mouse events aren't being dispatched for most of the skyline.

It's like starting CRD creates a dead zone at the top of the screen. I still have no explanation for this.

Was able to reproduce this with the initial user experience window, too. Meaning, when you log in for the first time, the first chrome window does not have a responsive tab switcher.

This does not reproduce with --mash or in classic ash. Just in --mus.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1672701ad759a71e686f41179cd21bf7991b7365

commit 1672701ad759a71e686f41179cd21bf7991b7365
Author: Elliot Glaysher <erg@chromium.org>
Date: Wed Dec 13 22:40:40 2017

Fix dead area after a chrome app is started in --mus.

After starting a chrome native app in --mus, the top 33 pixels of the
screen no longer accept mouse input. This was because code that is mash
only (which set the non-client area for a new window) was being run in
all mus contexts, meaning opening an app gave the ash RootWindow a
non-client area.

This was especially bad since a native welcome app is launched on first
login, meaning under --mus, you couldn't interact with the skyline
during your first session.

Bug:  794285 
Change-Id: I1dc69dc8fd48fb174ecc1db3198844a74b8a0e44
Reviewed-on: https://chromium-review.googlesource.com/825651
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elliot Glaysher <erg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523922}
[modify] https://crrev.com/1672701ad759a71e686f41179cd21bf7991b7365/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc