Has to do with the notch-doodle thing detecting that it is in motions, sometimes.
It doesn't happen every time.
Essentially it does into the "opening the launcher" animation.

I think this will be a simple check for tablet mode to prevent that animation.

That being said... checks for tablet mode aren't always simple.

Turns out that we had the peeking launcher's hinting arrow animation running eternally, even in tablet mode. That caused this to happen when you had the launcher pushed off the top for more than ten seconds.
That being said, it came right back when you left it in place for ten more seconds.

The fix for this is landing, I'll mark it fixed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d87f18eebc763c1f14e0735af3325595075d67f

commit 3d87f18eebc763c1f14e0735af3325595075d67f
Author: Kevin Strohbehn <ginko@google.com>
Date: Fri Oct 19 01:12:49 2018

Prevents hint animation from running eternally, even in tablet mode

Bug:  893218 
Change-Id: I95f07482e66f0e30d77c2ebda94f6a47f2eb86e7
Reviewed-on: https://chromium-review.googlesource.com/c/1290000
Commit-Queue: Kevin Strohbehn <ginko@google.com>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601006}
[modify] https://crrev.com/3d87f18eebc763c1f14e0735af3325595075d67f/ash/app_list/views/expand_arrow_view.cc