From this screenshot, it looks like they don't have enough comments to fill the pane, necessitating a scroll bar. I can confirm though that the scroll wheel does nothing when there is a large comment list and a scrollbar. Even if the scrollbar is active (clicked on) first. Same with hovering the mouse over the list or scrollbar. I'm using Acrobat XI Pro V 11.0.23.
From this screenshot, it looks like they don't have enough comments to fill the pane, necessitating a scroll bar. I can confirm though that the scroll wheel does nothing when there is a large comment list and a scrollbar. Even if the scrollbar is active (clicked on) first. Same with hovering the mouse over the list or scrollbar. I'm using Acrobat XI Pro V 11.0.23.