Remove absorbPanEventsOnScrollables option

[rorystephenson]

The only reason to disable absorbing of pan events is because some plugins which use drag gestures rely on it. It turns out if those plugins detect vertical/horizontal drag instead of pan this flag is not necessary.

Note that affected plugins need to be updated and this should be released in a major version bump as it is a breaking change.

See #1454 discussion leading to this PR.

Before releasing

• Notify affected plugin authors of the required change.
• Create new major version with changelog entry to describe how other plugins must migrate.

Understanding more

• When absorbPanEventsOnScrollables is true a horizontal & vertical drag gesture detector are added to FlutterMap and if they are triggered FlutterMap's scale gesture logic is triggered since the vertical/horizontal drag gesture and scale gesture are in the same GestureArenaTeam and the scale gesture is the captain of that team. Horizontal/vertical drag gestures are recognised faster than scale/pan gestures and therefore when there are competing gestures they will prevail.
• Scrollable Flutter widgets detect horizontal/vertical drag gestures which is why FlutterMap needs to detect them and then defer to its scale gesture if it wants to be pannable inside of a ListView.
• flutter_map_dragmarker and likely other plugins detect pan gestures instead of horizontal/vertical drag gestures. This means that when absorbPanEventsOnScrollables is enabled those plugins don't work because FlutterMap's horizontal/vertical drag gestures have precedence. This is a simple fix, affected code should listen to onVerticalDrag(Start/Update/End) and onHorizontalDrag(Start/Update/End) instead of onPan(Start/Update/End).

Demo

With absabsorbPanEventsOnScrollables removed such that horizontal/vertical gestures are always detected in FlutterMap (see this PR's code changes) and a modified flutter_map_dragmarker (detecting horizontal/vertical drag instead of pan) is used it is possible to have a pan-able FlutterMap inside of a ListView containing drag markers which can be dragged:

demo.mp4

[JaffaKetchup]

Thanks @rorystephenson, we'll try to have a look at this soon!

[TesteurManiak]

LGTM 👍

[JaffaKetchup]

@rorystephenson When you get a moment from working on #1475, can you resolve the conflicts here for merge? (Also note that #1475 needs the same conflicts resolved).

[rorystephenson]

@rorystephenson When you get a moment from working on #1475, can you resolve the conflicts here for merge? (Also note that #1475 needs the same conflicts resolved).

Rebased both 👍 :)

[JaffaKetchup]

@rorystephenson You happy for this to be merged now? Not sure if it will cause more conflicts.

[rorystephenson]

@rorystephenson You happy for this to be merged now? Not sure if it will cause more conflicts.

Yep I think it's good to go.