On Mac it is possible to scroll content in inactive windows (windows that don't have focus). For such windows glass generates ScrollStarted, then all the Scroll events, but no ScrollFinished and no subsequent ScrollStarted for the next gesture (although the Scroll events are still coming).

An easy way to reproduce: run HelloScrollPane, focus another application, scroll one of the scroll panes (by two-finger gesture on trackpad), then try to scroll another scroll pane - still the same pane moves, because Scenegraph thinks it is still the same scrolling gesture.