Here are two screenshots of the Settings app on my iPhone, from a couple of years ago.
Notice the dark gray scrollbar on the right of the first screenshot, indicating that more information is available with a downward swipe action. In the second screenshot, the scrollbar has faded away.
Now think about the context in which you typically use a mobile phone. Are you looking at the display the entire time? Not always. In this case, if you happen to look away from the phone for half a second, you might not see the scrollbar before it disappears. And the display is laid out so that there are no other visual cues about further settings being possible to access.
I can use this example in a few different contexts:
- To highlight a new-user perspective on the use of a device.
- To emphasize the importance of providing information to users about the state the system is in and what it's possible to do in that state.
- To observe that a cool effect doesn't always improve a system's usability.
- To open a discussion of differences between mobile and desktop/laptop applications: the mobile scrollbar is simply a visual indicator; it's not interactive in the way a traditional scrollbar is.
- To open a discussion of visual affordances.