Discussed offline:

Arrow keys should not autocomplete until further notice.

Currently pressing the 'down' arrow will solidify the autocomplete, and modify the results you were attempting to go through.

Does this bug cover fixing that?

Yes, it will.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/55b298b37b149a161c1643fc40b5dccb377d4a52

commit 55b298b37b149a161c1643fc40b5dccb377d4a52
Author: Daniel Zhang <oxyflush@google.com>
Date: Tue Aug 21 18:20:11 2018

cros: Remove AppList arrow key autocomplete.

Removing the completion of an autocomplete
suggestion with the arrow keys until further
UX input. Also updated tests and current code
to reflect this.

Bug:  874518 
Change-Id: I2d35799b8b50e93b466c82e7fba9c5c523e9bef8
Reviewed-on: https://chromium-review.googlesource.com/1180100
Commit-Queue: Daniel Zhang <oxyflush@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584835}
[modify] https://crrev.com/55b298b37b149a161c1643fc40b5dccb377d4a52/ash/app_list/views/search_box_view.cc
[modify] https://crrev.com/55b298b37b149a161c1643fc40b5dccb377d4a52/ash/app_list/views/search_box_view_unittest.cc

Verified on M-71 (11151.11.0, 71.0.3578.21).