Screenshot is below. Because some lines of code do not fit the view (both on the before and the after side of the diff), you would expect to see horizontal scrollbars; but they are not there.
While you can use a hacky workaround by enabling the "fit to screen" feature of Gerrit, the correct solution is to have scrollbars when you need them. This maybe an Upstream issue (I have no way to tell). It is not browser-specific and I could reproduce it in Chrome, Firefox, etc.
UPDATE: the upstream issue is https://bugs.chromium.org/p/gerrit/issues/detail?id=13132