FB-10: close the ambient-tick-vs-drag race that snapped the map back

A pointer down/up Listener around the FlutterMap now tracks whether a real
gesture is in progress, and didUpdateWidget's follow-recenter callbacks skip
the camera move while one is -- closing the timing window where a once-per-
second ambient GPS tick's postFrameCallback could land after a finger touched
the map but before flutter_map reported hasGesture: true, snapping the camera
back out from under an in-progress pan.

test/ride_map_test.dart

@@ -520,6 +520,79 @@ void main() {
       expect(center.longitude, closeTo(fix1.longitude, 1e-9));
     });
 
+    testWidgets(
+        'a real drag beats an ambient tick that lands mid-drag (FB-10)',
+        (tester) async {
+      const fix1 = ll.LatLng(51.0, -114.0);
+      // The ambient tick that lands while the finger is down but before
+      // flutter_map's own gesture recognizer has reported `hasGesture: true` --
+      // this is the exact race the ticket describes.
+      const fix2 = ll.LatLng(51.5, -114.5);
+
+      Widget build(ll.LatLng ambient) => MaterialApp(
+            theme: ripprTheme(),
+            home: Scaffold(
+              body: RideMap(
+                points: const [],
+                segments: const [],
+                showEmptyLabel: false,
+                follow: true,
+                ambientPosition: ambient,
+              ),
+            ),
+          );
+
+      await tester.pumpWidget(build(fix1));
+      await tester.pump();
+
+      // Put a real pointer down on the map -- this is what a rider's finger
+      // touching the screen looks like, well before flutter_map decides the
+      // movement counts as a drag.
+      final gesture =
+          await tester.startGesture(tester.getCenter(find.byType(FlutterMap)));
+      addTearDown(() => gesture.removePointer());
+
+      // Simulate a once-per-second ambient GPS tick landing while the pointer
+      // is already down but before it has moved -- exactly the race window
+      // the ticket describes: flutter_map has not yet reported `hasGesture:
+      // true`, so nothing has flipped `_following` off yet.
+      await tester.pumpWidget(build(fix2));
+      await tester.pump();
+
+      // The critical assertion: with the pointer still down and untouched by
+      // any real drag, the camera must not have snapped to the ambient tick's
+      // position. Without the fix, the race lets the scheduled
+      // `postFrameCallback` win here and the camera jumps to `fix2` before
+      // the rider's finger has moved at all.
+      var map = tester.widget<FlutterMap>(find.byType(FlutterMap));
+      var center = map.mapController!.camera.center;
+      expect(
+        center.latitude,
+        isNot(closeTo(fix2.latitude, 1e-6)),
+        reason: 'the mid-drag ambient tick must not win the race and snap the '
+            'camera back to the ambient position while the gesture is still '
+            'down',
+      );
+      expect(center.longitude, isNot(closeTo(fix2.longitude, 1e-6)));
+
+      // Now the finger actually moves and lifts -- the real drag completes
+      // normally, and the final position reflects it, not the ambient tick.
+      await gesture.moveBy(const Offset(-100, -100));
+      await tester.pump();
+      await gesture.up();
+      await tester.pump();
+
+      map = tester.widget<FlutterMap>(find.byType(FlutterMap));
+      center = map.mapController!.camera.center;
+      expect(
+        center.latitude,
+        isNot(closeTo(fix2.latitude, 1e-6)),
+        reason: 'the completed drag must still reflect the rider\'s own pan, '
+            'not the ambient position the mid-drag tick tried to recenter to',
+      );
+      expect(center.longitude, isNot(closeTo(fix2.longitude, 1e-6)));
+    });
+
     testWidgets('tapping recenter moves the camera to the latest recorded '
         'point when recording', (tester) async {
       final points = [for (var i = 0; i < 4; i++) p(1, i)];