Both Firefox and Chrome have useful debugging tools to help you find out more information about scripts relating to a behavior. I'll be explaining mostly in Chrome as their default debug tools are more extensive
As a start, you can usually use
Right Click->Inspect Element to find out information about an element quickly. However this does not appear to work on Facebook's ticker - doing so will select the "Ticker News" behind the scrollbar like this:
Because we cannot see the element being modified, we will have to make some guesses. The ticker stories themselves do not appear to have anything related to the scrollbars. After traveling up to the parent and folding
<div class="tickerActivityStories">, we can start to see things clearly
By hovering over (and invoking the scrollbars's "appear" code) we can observe the page's DOM and note the differences.
So it appears that
<div class="uiScrollableAreaTrack"> as it responds accordingly whenever the mouse enters/leaves. The name also makes sense (implies that it's the scrollbar's tracks). By unfolding that
div, we'll notice that there is even a "gripper" for the scrollbar!
<div class="uiScrollableAreaGripper", and on the right pane, unfold the
Event Listeners tab. We can assume that it responds to focus, and under the
If you're in luck, the files may have a recognizable name (such as
typeahead.min.js or similar, which you can google). Unfortunately in most cases, including ours, we are likely looking at proprietary code or minified code. (Minified code is extremely common to reduce file sizes so that pages load faster)