Version: 55.0.2845.0 (Canary); OK in 52.0.2743.116 (stable)
OS: Only tested on Mac so far

What steps will reproduce the problem?
(1) Visit Gmail. (You might need to be using an Apple trackpad/magic mouse.)
(2) Open a long-ish email.
(3) Play with scrolling with the mouse cursor over the email content.
(4) Play with scrolling with the mouse cursor over the sidebar on the right.

What is the expected output?
Scrolling speed is the same in both cases.

What do you see instead?
Scrolling speed is normal with the cursor over the content, but about 2x speed with the cursor over the sidebar.

Maybe they're using events to catch scroll wheel action over the sidebar and something changed there?