scroll: Apply a min velocity in ScrollToEndSimulation

This makes it possible to see in a self-contained way, in this class's
own code, that it always starts moving at a velocity that isn't zero,
or less than zero, or at risk of being conflated with zero.

This doesn't have a big effect in practice, because the only call site
already does something else whenever the distance to travel is
negative or very close to zero.  But there is a small range -- namely
where the distance to travel is between 1 and 12 physical pixels,
given the default behavior of ScrollPhysics.toleranceFor -- in which
this minimum speed does apply.

Author: gnprice

lib/widgets/scrolling.dart

@@ -364,12 +364,18 @@ class InertialSimulation extends Simulation { // TODO(upstream)
 /// Instead this takes a smoothed-out approximation of such a trajectory.
 class ScrollToEndSimulation extends InertialSimulation {
   factory ScrollToEndSimulation(ScrollPosition position) {
+    final tolerance = position.physics.toleranceFor(position);
     final startPosition = position.pixels;
     final estimatedEndPosition = position.maxScrollExtent;
     final velocityForMinDuration = (estimatedEndPosition - startPosition)
       / (minDuration.inMilliseconds / 1000.0);
-    assert(velocityForMinDuration > 0);
-    final velocity = clampDouble(velocityForMinDuration, 0, topSpeed);
+    final velocity = clampDouble(velocityForMinDuration,
+      // If the starting position is beyond the estimated end
+      // (i.e. `velocityForMinDuration < 0`), or very close to it,
+      // then move forward at a small positive velocity.
+      // Let the overscroll handling bring the position to exactly the end.
+      2 * tolerance.velocity,
+      topSpeed);
     return ScrollToEndSimulation._(startPosition, velocity);
   }
 

test/widgets/scrolling_test.dart

@@ -418,6 +418,27 @@ void main() {
         debugDefaultTargetPlatformOverride = null;
       });
 
+      testWidgets('starting very near end, apply min speed', (tester) async {
+        await prepare(tester, topHeight: 400, bottomHeight: 400);
+        // Verify the assumption used for constructing the example numbers below.
+        check(position.physics.toleranceFor(position).velocity)
+          .isCloseTo(20/3, .01);
+
+        position.jumpTo(398);
+        await tester.pump();
+        check(position.extentAfter).equals(2);
+
+        position.scrollToEnd();
+        await tester.pump();
+        check(position.extentAfter).equals(2);
+
+        // Reach the end in just 150ms, not 300ms.
+        await tester.pump(Duration(milliseconds: 75));
+        check(position.extentAfter).equals(1);
+        await tester.pump(Duration(milliseconds: 75));
+        check(position.extentAfter).equals(0);
+      });
+
       testWidgets('on overscroll, stop', (tester) async {
         debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
         await prepare(tester, topHeight: 400, bottomHeight: 1000);