Chrome Version: 65.0.3285.0 (Developer Build) (64-bit)
OS: Linux

What steps will reproduce the problem?
(1) Run chrome with --enable-features=ExtendedMouseButtons
(2) Navigate to example.com. Ensure that the back button is enabled on the browser.
(3) Add an event listener for mousedown by running the following in the console:

document.addEventListener('mousedown', (event)=>{event.preventDefault(); console.log(event.button); }, false);

(4) Hit F11 (assuming that preventDefault should only work in full-screen mode, as is the case for 
(5) Click the mouse "back" button.

What is the expected result?
3 should be logged to the console and no navigation should occur. This is my understanding based on the comments for https://chromium-review.googlesource.com/c/chromium/src/+/781999:

"Allow mousedown to prevent default the back/forward actions."

What happens instead?
3 is logged to the console (so I think I've enabled the feature correctly), but Chrome navigates back to the previous page.