When doing web programming in a context where scroll might be expected, you may see some surprising behavior in the scrollable HTML– an inexplicable gray bar on the bottom or right of the element.
One culprit, on Mac, is the user’s scrollbar setting.
As a UX and accessibility aid, Mac allows users to always show scrollbars when they are available. Considering this feature as a programmer, you can make sure you only offer scrolling when it makes sense, and educate users who don’t know they have this setting enabled.
See “Settings > Appearance > Show scroll bars” to see and adjust this setting.