If you use an invalid event pointer value (e.g., `activePointer = -1;`) as the "flag" for nothing being dragged, you can drop the `isDragging` boolean entirely.  Feel free to leave it as-is if you prefer the readability.

The rest looks fine to me, but the scene2d maintainers may have a better idea for handling this.  (But what you have looks similar to how `ClickListener` handles overlapping pointers.)