See https://bugs.chromium.org/p/gerrit/issues/detail?id=4292 for upstream bug report. Repro case in our test instance is http://gerrit-test.wmflabs.org/gerrit/#/c/32/2/tests/fixtures/layout-cloner.yaml . Make sure you're in side-by-side mode, and make the window narrow enough that line 7 on the right-hand side doesn't fit. The ); at the end will become inaccessible to the scroll bars.
This seems to be fixed in polygerrit. Gerrit's new ui. Seee https://go-review.googlesource.com/?polygerrit=1 and https://go-review.googlesource.com/#/c/21732/4/src/internal/trace/parser.go
It seems to be CodeMirror that is the problem.
We will need to manually patch gerrit with the change, but once we do that, we can send it upstream for them to review :)
This is https://github.com/gerrit-review/gerrit/blob/7f4c48f37b9a8970ae2f8c8dc881f603ec9e707f/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide.java#L254 what needs patching and I will need to get approval from @demon if we can do this please? Since we can send it upstream for reviews after :)