[Proposal] Make mouseWheel zoom in % instead of pixels value (#71266)

InteractiveViewer scale gesture feel improvement.

packages/flutter/lib/src/widgets/interactive_viewer.dart

@@ -930,12 +930,15 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
         widget.onInteractionEnd?.call(ScaleEndDetails());
         return;
       }
-      final RenderBox childRenderBox = _childKey.currentContext!.findRenderObject()! as RenderBox;
-      final Size childSize = childRenderBox.size;
-      final double scaleChange = 1.0 - event.scrollDelta.dy / childSize.height;
-      if (scaleChange == 0.0) {
+
+      // Ignore left and right scroll.
+      if (event.scrollDelta.dy == 0.0) {
         return;
       }
+
+      // In the Flutter engine, the mousewheel scrollDelta is hardcoded to 20 per scroll, while a trackpad scroll can be any amount.
+      // The calculation for scaleChange here was arbitrarily chosen to feel natural for both trackpads and mousewheels on all platforms.
+      final double scaleChange = math.exp(-event.scrollDelta.dy / 200);
       final Offset focalPointScene = _transformationController!.toScene(
         event.localPosition,
       );

packages/flutter/test/widgets/interactive_viewer_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
+
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
@@ -673,6 +675,37 @@ void main() {
       expect(scenePoint, const Offset(100, 100));
     });
 
+     testWidgets('Scaling amount is equal forth and back with a mouse scroll', (WidgetTester tester) async {
+      final TransformationController transformationController = TransformationController();
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+              body: Center(
+            child: InteractiveViewer(
+              constrained: false,
+              maxScale: 100000,
+              minScale: 0.01,
+              transformationController: transformationController,
+              child: Container(width: 1000.0, height: 1000.0),
+            ),
+          )),
+        ),
+      );
+
+      final Offset center = tester.getCenter(find.byType(InteractiveViewer));
+      await scrollAt(center, tester, const Offset(0.0, -200.0));
+      await tester.pumpAndSettle();
+      expect(transformationController.value.getMaxScaleOnAxis(), math.exp(200 / 200));
+      await scrollAt(center, tester, const Offset(0.0, -200.0));
+      await tester.pumpAndSettle();
+      // math.exp round the number too short compared to the one in transformationController.
+      expect(transformationController.value.getMaxScaleOnAxis(), closeTo(math.exp(400 / 200), 0.000000000000001));
+      await scrollAt(center, tester, const Offset(0.0, 200.0));
+      await scrollAt(center, tester, const Offset(0.0, 200.0));
+      await tester.pumpAndSettle();
+      expect(transformationController.value.getMaxScaleOnAxis(), 1.0);
+    });
+
     testWidgets('onInteraction can be used to get scene point', (WidgetTester tester) async{
       final TransformationController transformationController = TransformationController();
       late Offset focalPoint;