FYI: I don't see this issue on the official build (version 64.0.3242.0 canary).

@yhanada, could you let us know the commit hash where you saw this issue?

I saw this on c452aca7b17c8938d24dbc0a17490eb01b9cb065.

As I checked, no suspicious commit in ui/arc and ui/message_center.

https://chromium.googlesource.com/chromium/src/+log/4840cb96f11c8a472c19a1f75027f8ac1800a6df..f2c191bfb11db948394f065cd25fa9bcd84626e9/ui/message_center
https://chromium.googlesource.com/chromium/src/+log/4840cb96f11c8a472c19a1f75027f8ac1800a6df..f2c191bfb11db948394f065cd25fa9bcd84626e9/ui/arc

It looks an other view change may affect this.

Ah, after I synced to ToT, the issue is gone. Sorry for the confusion!