It's a little hard to say with the limited information you provided. What javascript / plugin do you use to achieve infinite scroll?
My gut instinct is that an infinite scroll can occur in the element below the top level html element. Perhaps try using the browser developer tools to determine which item is actually scrolling, and applying "overflow-y: scrolling"; to that.
Be careful, you may find that the reason they hide the vertical scrollbar is because the plugin you are using makes it meaningless. If the system removes old items from the top of the page to run out of memory, scrolling at the top of the scroll bar may not bring you to the top of the page.
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