I didn’t notice much since generally don’t have the arrow buttons and I wouldn’t use them. I use arrow keys, pagination keys, home/end key, scroll wheel/motion, drag the bar or click somewhere to jump there. Those buttons were always quite tiny.
But the behavior of my scrollbar looks like this: slides in on use or when the mouse gets moved; gets fatter when hovered
Though hiding stuff sucks indeed.
Edit: You can configure scrollbar to always be visible.