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.