WebKit/Blink's (Safari/Chrome) default behaviour on MacOS since 10.7 (Mac OS X Lion) is to hide scroll bars from trackpad users when they're not in use. This can be confusing; the scroll bar is often the only visual cue that an element is scrollable.
Example (jsfiddle)
HTML<div class="frame">
    Foo<br />
    Bar<br />
    Baz<br />
    Help I'm trapped in an HTML factory! 
</div>
.frame {
    overflow-y: auto;
    border: 1px solid black;
    height: 3em;
    width: 10em;
    line-height: 1em;
}

 
 
How can I force a scroll bar to always be displayed on a scrollable element in WebKit?
 
     
 
 
     
     
     
     
    