placebo feature, it has always been that double tap on anywhere on screen does the zoomin/zoomout (in addition to pinch gesture). Getting scrollbars to show with dragging gesture on the screen (and then using swipe up and down) has always been there, you just happen to init dragging gesture (to show scrollbars) with tap/double tap sometimes. PS. userguide page 41, but most people don't read the manuals.