handleThumbPressStartHorizontal method
Handler called when a long press gesture has started on the horizontal scrollbar.
Begins the fade out animation and initializes dragging the scrollbar thumb.
Implementation
@override
void handleThumbPressStartHorizontal(Offset position) {
super.handleThumbPressStartHorizontal(position);
_dragIsActiveH = true;
updateScrollbarPainter(false);
}