I confirmed this happens on 68.0.3424.0.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/004d768e0d37003e65cfc7e4ab8c7b9a2ad33666

commit 004d768e0d37003e65cfc7e4ab8c7b9a2ad33666
Author: Tatsuhisa Yamaguchi <yamaguchi@google.com>
Date: Tue May 08 07:22:28 2018

Limit web notification bubble height to fit in the user work area.

Change-Id: I1b8f6ab60fd5c871e0c119200a01f4ce091fdd7c
Bug:  833826 
Test: Manually verified by turning on docked magnifier.
Reviewed-on: https://chromium-review.googlesource.com/1049466
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556714}
[modify] https://crrev.com/004d768e0d37003e65cfc7e4ab8c7b9a2ad33666/ash/system/web_notification/web_notification_tray.cc

Yamaguchi-san, thank you very much!

Sorry, one more thing. Could you merge this to M67?

This bug requires manual review: M67 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

#1 says this is a non-regressing meaning it was in place before M67?

We typically don't merge this late for UI/cosmetic issues. Declining; please re-tag for merge with reasoning if required.

Thanks for explanation. I think we can have this fix at M68.

I think this issue can only happen after introduction of docked magnifier, which seems to have been available from M66. I haven't confirmed if this was happening on M66, though.