They're using ::-webkit-scrollbar
and the associated pseudo-elements, which only work in WebKit browsers (which is fine, because this is just aesthetics).
Take a look at this for more information: Apple-like scrollbars using CSS
I've taken the CSS that Twitter is using, see: http://jsbin.com/ubasew
#doc ::-webkit-scrollbar{width:9px;height:9px;}
#doc ::-webkit-scrollbar-button:start:decrement,#doc ::-webkit-scrollbar-button:end:increment{display:block;height:0;background-color:transparent;}
#doc ::-webkit-scrollbar-track-piece{background-color:#FAFAFA;-webkit-border-radius:0;-webkit-border-bottom-right-radius:8px;-webkit-border-bottom-left-radius:8px;}
#doc ::-webkit-scrollbar-thumb:vertical{height:50px;background-color:#999;-webkit-border-radius:8px;}
#doc ::-webkit-scrollbar-thumb:horizontal{width:50px;background-color:#999;-webkit-border-radius:8px;}
The #doc
is as Twitter had it, and it's there so that only scrollbars inside #doc
are customised.
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