The thing that puzzles me is that I would expect overflow-x: hidden to result in text typed into that textarea being wider than the page and being invisibly truncated on the right hand side.
But that's not what happens. And in fact, when you start typing in the textarea the horizontal scrollbar vanishes - it's only there when the textarea is empty.
Am I misunderstanding anything here? Seems like it's some weird Safari bug, since Firefox and Chrome don't have the problem.
It probably has to do with other styles assigned to the textarea, maybe the ::placeholder as it hides when typing (I assume on focus)
In any case. In the screenshot the scrollbar is inside the textarea as it aligns with the resize control on its right. This is basically all the info needed to deduce the textarea overflow is the culprit.
But could be that the overflow-x is just a bandaid hiding the issue causing the overflow in the first place, like crazy styles on the placeholder.