Interesting... I downloaded dataset-agent and removed various different styles from the textarea (with an intention of providing a PR) including the overflow-x: hidden and I tried Safari and Chrome with both the global Mac setting of Always showing scrollbars on and off. It NEVER shows the scrollbar for me.

Do you have an extension installed that is doing something weird to your textareas? Maybe I'm doing it wrong but I think for now overflow-x is fine if you are experiencing it and I am not! Let's all get on with our lives... I was probably a bit overzealous about caring all that much about a perfectly fine CSS fix.