The behaviour is different in Epy and MB, however is faulty in both cases. Epy --- Steps to reproduce: 1- Open https://community-tc.services.mozilla.com/tasks/UAWFTd_hTIGeOrx2SQsDuw/runs/0/logs/https%3A%2F%2Fcommunity-tc.services.mozilla.com%2Fapi%2Fqueue%2Fv1%2Ftask%2FUAWFTd_hTIGeOrx2SQsDuw%2Fruns%2F0%2Fartifacts%2Fpublic%2Flogs%2Flive.log 2- Pay attention to the vertical scrollbar. It's duplicated. Using the mouse wheel allows vertical scrolling. The small scrollbar moves but it's impossible to click on it. MB -- 1- Open the above URL 2- Only the small vertical scrollbar is rendered. However is impossible to click and/or interact with it.