[Material] Remove "down position" from toggleable ripple calculation (#112209)

* remove down position from toggleable ripple

Author: johnsonmh

packages/flutter/lib/src/material/toggleable.dart

@@ -559,7 +559,6 @@ abstract class ToggleablePainter extends ChangeNotifier implements CustomPainter
           focusColor,
           reactionFocusFade.value,
         )!;
-      final Offset center = Offset.lerp(downPosition ?? origin, origin, reaction.value)!;
       final Animatable<double> radialReactionRadiusTween = Tween<double>(
         begin: 0.0,
         end: splashRadius,
@@ -568,7 +567,7 @@ abstract class ToggleablePainter extends ChangeNotifier implements CustomPainter
           ? splashRadius
           : radialReactionRadiusTween.evaluate(reaction);
       if (reactionRadius > 0.0) {
-        canvas.drawCircle(center + offset, reactionRadius, reactionPaint);
+        canvas.drawCircle(origin + offset, reactionRadius, reactionPaint);
       }
     }
   }

packages/flutter/test/material/checkbox_test.dart

@@ -602,6 +602,38 @@ void main() {
     );
   });
 
+  testWidgets('Checkbox starts the splash in center, even when tap is on the corner', (WidgetTester tester) async {
+    Widget buildApp() {
+      return MaterialApp(
+        theme: theme,
+        home: Material(
+          child: Center(
+            child: StatefulBuilder(builder: (BuildContext context, StateSetter setState) {
+              return Checkbox(
+                value: false,
+                onChanged: (bool? newValue) {},
+              );
+            }),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildApp());
+    final Offset checkboxTopLeftGlobal = tester.getTopLeft(find.byType(Checkbox));
+    final Offset checkboxCenterGlobal = tester.getCenter(find.byType(Checkbox));
+    final Offset checkboxCenterLocal = checkboxCenterGlobal - checkboxTopLeftGlobal;
+    await tester.startGesture(checkboxTopLeftGlobal);
+    await tester.pump();
+    // Wait for the splash to be drawn, but not long enough for it to animate towards the center, since
+    // we want to catch it in its starting position.
+    await tester.pump(const Duration(milliseconds: 1));
+    expect(
+      Material.of(tester.element(find.byType(Checkbox))),
+      paints..circle(x: checkboxCenterLocal.dx, y: checkboxCenterLocal.dy),
+    );
+  });
+
   testWidgets('Checkbox can be hovered and has correct hover color', (WidgetTester tester) async {
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
     bool? value = true;