The scrollbar drag engine — the 1-D analog of createDragCells. One pointer surface (the track)
serves all three gestures: thumb drag, track page-and-hold, and the parked-at-edge hold. The engine
owns the mechanics — pointer capture, zone hit-testing, one hold timer, last-point re-apply — and has
NO scroll policy: an TrackDragOptions.onStart adapter supplies it as a
TrackDragSession, so a linear and a virtual/elastic scrollbar differ only in the session.
The scrollbar drag engine — the 1-D analog of
createDragCells. One pointer surface (the track) serves all three gestures: thumb drag, track page-and-hold, and the parked-at-edge hold. The engine owns the mechanics — pointer capture, zone hit-testing, one hold timer, last-point re-apply — and has NO scroll policy: an TrackDragOptions.onStart adapter supplies it as a TrackDragSession, so a linear and a virtual/elastic scrollbar differ only in the session.