In Windows the additional mouse buttons are often handled by some vendor app that comes with the driver. 
It intercepts such clicks and blindly sends a hotkey like Alt-Left to navigate back.

I added a mousesdown and mouseup listener on the page and it does should capture events with mouse button 3.  So it looks like drivers are bit eating the button events and they are reaching Chrome.

Not sure how I can check whether driver is also synthesizing key input.

I'm not able to repro this on my workstation (Win10 64-bit, 68.0.3440.84 (Official Build) (64-bit)).

I'm using a Logitech mouse (M-U0007) with no Logitech drivers installed. Maybe this is dependent on the vendor app as suggested by woxxom@?

I tried reproing again after installing SetPoint. With the default configuration, Forward and Back actions are mapped to the side buttons. Chrome still sees the mouse events and navigation is blocked.

Sorry, we were missing some info in the repro.

It seems related to pointer lock.

When point lock is requested, the preventDefault() does not seem to work.

A simple repro is to go to the MDN pointer lock sample page (https://mdn.github.io/dom-examples/pointer-lock/).

Register the listener on this page.
Enter pointer lock.
Then press button 3.

This appears to be the same pointer lock related issue from  crbug.com/852709 , merging into that bug.

Let's see if we can get this merged for M68.