UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3355.0 Safari/537.36

Example URL:
https://joinmastodon.org/

Steps to reproduce the problem:
1) Connect Macbook Pro to an external monitor over DisplayPort
2) Open joinmastodon.org on laptop monitor
3) Open the "I am everything" drop-down (several screens down)
4) Position mouse over the items in the menu that pops down
5) Use the scroll wheel
6) As expected, the menu scrolls

Now, drag the window to the secondary (DisplayPort) monitor and:
7) Position mouse over the items in the menu that pops down
8) Use the scroll wheel

What is the expected behavior?
The menu scrolls, like it did on the primary monitor

What went wrong?
The entire page scrolls, and the menu does not scroll.

In fact, this applies to every scrolling view on this page (including the list of social networks just below the drop-down, for example) and on every other site.

The only condition I've found under which the mouse wheel can be used to scroll nested views on my second monitor is if the outer page is scrolled near the top of the page. After you scroll down about half a screen, scroll events that should go to nested views scroll the page instead.

Also, on the second monitor, the scroll bars for the nested views are always visible, whereas on my primary monitor, they're only visible while I'm scrolling them.

Does it occur on multiple sites: Yes

Is it a problem with a plugin? No 

Did this work before? Yes 

Does this work in other browsers? Yes

Chrome version: 66.0.3355.0  Channel: n/a
OS Version: OS X 10.12.6
Flash Version: 

Laptop:
macOS 10.12.6
NVIDIA GeForce GT 750M 2048 MB, Intel Iris Pro 1536 MB

External monitor (connected via mini DisplayPort to DisplayPort):
Predator
3440 x 1440 @ 100 Hz