An online game for example.

You wouldn't want to hide scrollbars though.

You'd want to make it so either:

a) There is nothing to scroll in the first place, so you don't need a scrollbar

b) Capture the pointer so all interaction only occurs within the game's area.

Sometimes things overflow and you don't want scrollbars popping in and out.

Sure. I was thinking of fairly narrow cases, not ordinary page scrolling:

- A swipe/drag carousel with visible arrows or dots, especially a looping carousel where the native scrollbar does not represent a meaningful start and end

- An accordion or expanding panel that animates to reveal all of its content. During the transition, its intermediate height may briefly produce a scrollbar (and a visual flash), even though the fully expanded state does not need one

- Highly visual or experimental interfaces where scrolling is communicated through another control, or where the scrollable area is decorative and scrolling is not required to access the content