OnScrollEvent
OnScrollEvent class has the following properties:
direction
 The direction in which the user is scrolling. Has a value only when event_type="user".
Can be one of the following:
- "idle"
- "forward"
- "reverse"
event_type
 A string representing the type of the scroll event that occurred. Can be one of the following:
- "start"- control has started scrolling;
- "update"- control has changed its scroll position;
- "end"- control has stopped scrolling;
- "user"- user has changed the direction in which they are scrolling;
- "over"- control has not changed its scroll position because the change would have caused its scroll position to go outside its scroll bounds.
pixels
 The current scroll position, in logical pixels.
min_scroll_extent
 The minimum in-range value for pixels.
max_scroll_extent
 The maximum in-range value for pixels.
viewport_dimension
 The extent of the viewport.
scroll_delta
 The distance by which the scrollable was scrolled, in logical pixels. Has a value only when event_type="update".
overscroll
 The number of logical pixels that the scrollable avoided scrolling. Has a value only when event_type="over".
velocity
 The velocity at which the ScrollPosition was changing when this overscroll happened. Has a value only when event_type="over".