The Scrollable snapOffset() callback could allow snapping "back" #6427
Description
Currently, if a Scrollable's snapOffsetCallback() returns a value that effectively reverses the fling's direction, the fling simulation asserts. For applications using the snapOffsetCallback() to scroll to page boundaries, it would be useful to allow the callback's value to reverse the fling's direction.
I/flutter : ══╡ EXCEPTION CAUGHT BY GESTURE LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter : The following assertion was thrown while routing a pointer event:
I/flutter : 'package:flutter/src/physics/friction_simulation.dart': Failed assertion: line 39 pos 12:
I/flutter : 'startVelocity == 0.0 || endVelocity == 0.0 || startVelocity.sign == endVelocity.sign' is not true.
I/flutter : When the exception was thrown, this was the stack:
I/flutter : #2 FrictionSimulation.FrictionSimulation.through (package:flutter/src/physics/friction_simulation.dart:39:12)
I/flutter : #3 _createSnapScrollSimulation (package:flutter/src/widgets/scroll_behavior.dart:16:14)
I/flutter : #4 OverscrollBehavior.createSnapScrollSimulation (package:flutter/src/widgets/scroll_behavior.dart:257:12)
I/flutter : #5 ScrollableState._createSnapSimulation (package:flutter/src/widgets/scrollable.dart:622:50)
I/flutter : #6 ScrollableState._startToEndAnimation (package:flutter/src/widgets/scrollable.dart:587:19)
I/flutter : #7 ScrollableState.fling (package:flutter/src/widgets/scrollable.dart:564:14)