Fix: touches are ignored at top edge of list view (#228)

## Related issues (optional)

Fixes #212.

## Description

This PR fixes a problem related to floating-point precision errors in
the same manner as #209, taking the top edge of scrollable widgets into
account (the previous fix addressed only the bottom edge).

## Summary (check all that apply)

- [x] Modified / added code
- [x] Modified / added tests
- [ ] Modified / added examples
- [ ] Modified / added others (pubspec.yaml, workflows, etc...)
- [ ] Updated README
- [ ] Contains breaking changes
  - [ ] Created / updated migration guide
- [ ] Incremented version number
  - [ ] Updated CHANGELOG

lib/src/internal/float_comp.dart

@@ -74,4 +74,16 @@ class FloatComp {
 
   /// Returns [b] if [a] is approximately equal to [b], otherwise [a].
   double roundToIfApprox(double a, double b) => isApprox(a, b) ? b : a;
+
+  /// Rounds the given values to the nearest edge value if
+  /// they are approximately equal.
+  ///
+  /// If `a` is approximately equal to `b`, returns `b`.
+  /// If `a` is approximately equal to `c`, returns `c`.
+  /// Otherwise, returns `a`.
+  double roundToEdgeIfApprox(double a, double b, double c) {
+    if (isApprox(a, b)) return b;
+    if (isApprox(a, c)) return c;
+    return a;
+  }
 }

lib/src/scrollable/scrollable_sheet_extent.dart

@@ -208,18 +208,19 @@ class ScrollableSheetExtent extends SheetExtent
         draggableDistance + scrollableDistance;
     final scrollMetricsForScrollPhysics = scrollPosition.copyWith(
       minScrollExtent: 0,
-      // How many pixels the user can scroll and drag
+      // How many pixels the user can scroll and drag.
       maxScrollExtent: maxScrollExtentForScrollPhysics,
-      // How many pixels the user has scrolled and dragged
-      pixels: FloatComp.distance(context.devicePixelRatio).roundToIfApprox(
-        // Round the scrollPixelsForScrollPhysics to the maxScrollExtent if
-        // necessary to prevents issues with floating-point precision errors.
-        // For example, issue #207 was caused by infinite recursion of
+      // How many pixels the user has scrolled and dragged.
+      pixels: FloatComp.distance(context.devicePixelRatio).roundToEdgeIfApprox(
+        // Round the scrollPixelsForScrollPhysics to 0.0 or the maxScrollExtent
+        // if necessary to prevents issues with floating-point precision errors.
+        // For example, issue #207 and #212 were caused by infinite recursion of
         // SheetContentScrollPositionOwner.goBallisticWithScrollPosition calls,
         // triggered by ScrollMetrics.outOfRange always being true in
         // ScrollPhysics.createBallisticSimulation due to such a floating-point
         // precision error.
         scrollPixelsForScrollPhysics,
+        0,
         maxScrollExtentForScrollPhysics,
       ),
     );

test/scrollable/scrollable_sheet_test.dart

@@ -339,13 +339,16 @@ void main() {
     });
   });
 
-  // Regression test for https://github.com/fujidaiti/smooth_sheets/issues/207
-  testWidgets('Infinite ballistic scroll activity test', (tester) async {
+  // Regression tests for:
+  // - https://github.com/fujidaiti/smooth_sheets/issues/207
+  // - https://github.com/fujidaiti/smooth_sheets/issues/212
+  group('Infinite ballistic scroll activity test', () {
     late ScrollController scrollController;
     late ScrollableSheetExtent sheetExtent;
+    late Widget testWidget;
 
-    await tester.pumpWidget(
-      ScrollableSheet(
+    setUp(() {
+      testWidget = ScrollableSheet(
         child: Builder(
           builder: (context) {
             scrollController = PrimaryScrollController.of(context);
@@ -360,25 +363,47 @@ void main() {
             );
           },
         ),
-      ),
-    );
+      );
+    });
 
-    scrollController.jumpTo(600.0);
-    await tester.pumpAndSettle();
-    expect(scrollController.position.extentAfter, 0,
-        reason: 'Ensure that the scroll view cannot be scrolled anymore');
-
-    // Start a ballistic animation from a position extremely close to,
-    // but not equal, to the current position.
-    scrollController.position.correctPixels(600.000000001);
-    sheetExtent.goBallisticWithScrollPosition(
-      velocity: 0,
-      scrollPosition: scrollController.position as SheetContentScrollPosition,
-    );
-    await tester.pumpAndSettle();
-    expect(scrollController.position.pixels, 600.0);
-    expect(sheetExtent.activity, isA<IdleSheetActivity>(),
-        reason: 'Should not enter an infinite recursion '
-            'of BallisticScrollDrivenSheetActivity');
+    testWidgets('top edge', (tester) async {
+      await tester.pumpWidget(testWidget);
+      await tester.pumpAndSettle();
+      expect(scrollController.position.pixels, 0);
+
+      // Start a ballistic animation from a position extremely close to,
+      // but not equal, to the initial position.
+      scrollController.position.correctPixels(-0.000000001);
+      sheetExtent.goBallisticWithScrollPosition(
+        velocity: 0,
+        scrollPosition: scrollController.position as SheetContentScrollPosition,
+      );
+      await tester.pumpAndSettle();
+      expect(scrollController.position.pixels, 0);
+      expect(sheetExtent.activity, isA<IdleSheetActivity>(),
+          reason: 'Should not enter an infinite recursion '
+              'of BallisticScrollDrivenSheetActivity');
+    });
+
+    testWidgets('bottom edge', (tester) async {
+      await tester.pumpWidget(testWidget);
+      scrollController.jumpTo(600.0);
+      await tester.pumpAndSettle();
+      expect(scrollController.position.extentAfter, 0,
+          reason: 'Ensure that the scroll view cannot be scrolled anymore');
+
+      // Start a ballistic animation from a position extremely close to,
+      // but not equal, to the current position.
+      scrollController.position.correctPixels(600.000000001);
+      sheetExtent.goBallisticWithScrollPosition(
+        velocity: 0,
+        scrollPosition: scrollController.position as SheetContentScrollPosition,
+      );
+      await tester.pumpAndSettle();
+      expect(scrollController.position.pixels, 600.0);
+      expect(sheetExtent.activity, isA<IdleSheetActivity>(),
+          reason: 'Should not enter an infinite recursion '
+              'of BallisticScrollDrivenSheetActivity');
+    });
   });
 }