Chrome Version: 70.0.3536.0

Steps To Reproduce:
(1) open tests.peter.sh/notification-generator/ and display a notification
(2) open uber tray
(3) hover mouse on the top-right of the notification. Observe the close button.

Expected Result:
Close button displays inside the bubble

Actual Result:
See the screenshot in crbug.com/878772.
The square backdrop of the close icon gets out of the rounded corner.

How frequently does this problem reproduce? (Always, sometimes, hard to
reproduce?)
Always

What is the impact to the user, and is there a workaround? If so, what is
it?
This is a simple issue with UI appearance, not function.