Niconico's unique commenting system, also known as scrolling comments or danmaku (not to be confused with our actual danmaku tag). Nicovideo comments are displayed as floating text that scrolls over the video, from right to left. This leads to comments flooding the screen when users react to big moments.
Users can customize the time that their comments appear, or they can make their comments stay on the top or bottom of the screen for 3 seconds.