When I teach over Zoom, I have a small chat window on the side. It is very difficult to use because the vertical scrollbar keeps disappearing. I have to fidget with the size of the window in order to make the scroll bar appear, then I must move my cursor fast enough to grab the scroll bar before it disappears. Very frustrating.
My mac laptop had a similar "feature" of scroll bar disappearance, but I found a way to configure my laptop to keep the scroll bars. I do not know how to do it for the Zoom-generated chat box.