UPDATE:- This appears to be happending intermittently. Sometimes the horizontal scroll bar will appear when required, other times not.
I am currently working on a flow with a few CASE and CONDITION statements. This results in a very horizontally 'wide' flow. There is no horizontal scroll bar showing in my browser (Chrome) making it hard to navigate around the flow. I can successfully scroll horizontally using the arrow keys.
We recently made a change to work better with wider flows: https://flow.microsoft.com/blog/rich-approvals-text-and-multiselect/