This one's actually a workaround for a bug in Android 4.4 (definitely on Google's end as it's in their bug database with several dozen comments) - basically, the only way to get reliable access to the thumb scroller on 4.4 is to set it to be visible all the time (rather than having it only appear while you're scrolling). Unfortunately they don't seem to have fixed it in the finished version of Android 5.0 either, but perhaps they'll finally get around to dealing with it in 5.1.