Fix `Slider` `onChanged` callback order & never calls `onChangeStart` on ...

Fix `Slider` `onChanged` callback order & never calls `onChangeStart` on  `SliderInteraction.slideOnly` allowed interaction (#136720)

fixes [Slider will call onChanged before onChangeStart when sliding.](https://github.com/flutter/flutter/issues/136707)

This fixes a regression from https://github.com/flutter/flutter/pull/121483

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Example(),
    );
  }
}

class Example extends StatefulWidget {
  const Example({super.key});

  @override
  State<Example> createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  double _sliderValue = 0.5;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Slider(
          // allowedInteraction: SliderInteraction.tapAndSlide,
          // allowedInteraction: SliderInteraction.tapOnly,
          // allowedInteraction: SliderInteraction.slideOnly
          // allowedInteraction: SliderInteraction.slideThumb,
          value: _sliderValue,
          onChangeStart: (newValue) {
            print("onChangeStart ......");
          },
          onChanged: (newValue) {
            print("onChanged ......");
            setState(() {
              _sliderValue = newValue;
            });
          },
          onChangeEnd: (newValue) {
            print("onChangeEnd ......");
          },
        ),
      ),
    );
  }
}
```

</details>

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

@@ -1494,14 +1494,13 @@ class _RenderSlider extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
         case SliderInteraction.tapOnly:
           _active = true;
           _currentDragValue = _getValueFromGlobalPosition(globalPosition);
-          onChanged!(_discretize(_currentDragValue));
         case SliderInteraction.slideThumb:
           if (_isPointerOnOverlay(globalPosition)) {
             _active = true;
             _currentDragValue = value;
           }
         case SliderInteraction.slideOnly:
-          break;
+          onChangeStart?.call(_discretize(value));
       }
 
       if (_active) {
@@ -1509,6 +1508,7 @@ class _RenderSlider extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
         // a tap, it consists of a call to onChangeStart with the previous value and
         // a call to onChangeEnd with the new value.
         onChangeStart?.call(_discretize(value));
+        onChanged!(_discretize(_currentDragValue));
         _state.overlayController.forward();
         if (showValueIndicator) {
           _state.valueIndicatorController.forward();

packages/flutter/test/material/slider_test.dart

@@ -3886,6 +3886,7 @@ void main() {
       // (slider's left padding (overlayRadius), windowHeight / 2)
       const Offset startOfTheSliderTrack = Offset(24, 300);
       const Offset centerOfTheSlideTrack = Offset(400, 300);
+      final List<String> logs = <String>[];
 
       Widget buildWidget() => MaterialApp(
         home: Material(
@@ -3895,11 +3896,18 @@ void main() {
                 value: value,
                 key: sliderKey,
                 allowedInteraction: SliderInteraction.tapOnly,
+                onChangeStart: (double newValue) {
+                  logs.add('onChangeStart');
+                },
                 onChanged: (double newValue) {
+                  logs.add('onChanged');
                   setState(() {
                     value = newValue;
                   });
                 },
+                onChangeEnd: (double newValue) {
+                  logs.add('onChangeEnd');
+                },
               );
             }),
           ),
@@ -3909,26 +3917,35 @@ void main() {
       // allow tap only
       await tester.pumpWidget(buildWidget());
 
+      expect(logs, isEmpty);
+
       // test tap
       final TestGesture gesture = await tester.startGesture(centerOfTheSlideTrack);
       await tester.pump();
       // changes from 1.0 -> 0.5
       expect(value, 0.5);
+      expect(logs, <String>['onChangeStart', 'onChanged']);
 
       // test slide
       await gesture.moveTo(startOfTheSliderTrack);
       await tester.pump();
       // has no effect, remains 0.5
       expect(value, 0.5);
+      expect(logs, <String>['onChangeStart', 'onChanged']);
+
+      await gesture.up();
+      await tester.pump();
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChangeEnd']);
     });
 
-    testWidgetsWithLeakTracking('SliderInteraction.tapAndSlide', (WidgetTester tester) async {
+    testWidgetsWithLeakTracking('SliderInteraction.tapAndSlide (default)', (WidgetTester tester) async {
       double value = 1.0;
       final Key sliderKey = UniqueKey();
       // (slider's left padding (overlayRadius), windowHeight / 2)
       const Offset startOfTheSliderTrack = Offset(24, 300);
       const Offset centerOfTheSlideTrack = Offset(400, 300);
       const Offset endOfTheSliderTrack = Offset(800 - 24, 300);
+      final List<String> logs = <String>[];
 
       Widget buildWidget() => MaterialApp(
         home: Material(
@@ -3937,12 +3954,18 @@ void main() {
               return Slider(
                 value: value,
                 key: sliderKey,
-                // allowedInteraction: SliderInteraction.tapAndSlide, // default
+                onChangeStart: (double newValue) {
+                  logs.add('onChangeStart');
+                },
                 onChanged: (double newValue) {
+                  logs.add('onChanged');
                   setState(() {
                     value = newValue;
                   });
                 },
+                onChangeEnd: (double newValue) {
+                  logs.add('onChangeEnd');
+                },
               );
             }),
           ),
@@ -3951,11 +3974,14 @@ void main() {
 
       await tester.pumpWidget(buildWidget());
 
+      expect(logs, isEmpty);
+
       // Test tap.
       final TestGesture gesture = await tester.startGesture(centerOfTheSlideTrack);
       await tester.pump();
       // changes from 1.0 -> 0.5
       expect(value, 0.5);
+      expect(logs, <String>['onChangeStart', 'onChanged']);
 
       // test slide
       await gesture.moveTo(startOfTheSliderTrack);
@@ -3966,6 +3992,12 @@ void main() {
       await tester.pump();
       // changes from 0.0 -> 1.0
       expect(value, 1.0);
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChanged', 'onChanged']);
+
+      await gesture.up();
+      await tester.pump();
+
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChanged', 'onChanged', 'onChangeEnd']);
     });
 
     testWidgetsWithLeakTracking('SliderInteraction.slideOnly', (WidgetTester tester) async {
@@ -3975,6 +4007,7 @@ void main() {
       const Offset startOfTheSliderTrack = Offset(24, 300);
       const Offset centerOfTheSlideTrack = Offset(400, 300);
       const Offset endOfTheSliderTrack = Offset(800 - 24, 300);
+      final List<String> logs = <String>[];
 
       Widget buildApp() {
         return MaterialApp(
@@ -3985,11 +4018,18 @@ void main() {
                   value: value,
                   key: sliderKey,
                   allowedInteraction: SliderInteraction.slideOnly,
+                  onChangeStart: (double newValue) {
+                    logs.add('onChangeStart');
+                  },
                   onChanged: (double newValue) {
+                    logs.add('onChanged');
                     setState(() {
                       value = newValue;
                     });
                   },
+                  onChangeEnd: (double newValue) {
+                    logs.add('onChangeEnd');
+                  },
                 );
               }),
             ),
@@ -3999,11 +4039,14 @@ void main() {
 
       await tester.pumpWidget(buildApp());
 
+      expect(logs, isEmpty);
+
       // test tap
       final TestGesture gesture = await tester.startGesture(centerOfTheSlideTrack);
       await tester.pump();
       // has no effect as tap is disabled, remains 1.0
       expect(value, 1.0);
+      expect(logs, <String>['onChangeStart']);
 
       // test slide
       await gesture.moveTo(startOfTheSliderTrack);
@@ -4014,6 +4057,12 @@ void main() {
       await tester.pump();
       // changes from 0.0 -> 1.0
       expect(value, 1.0);
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChanged']);
+
+      await gesture.up();
+      await tester.pump();
+
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChanged', 'onChangeEnd']);
     });
 
     testWidgetsWithLeakTracking('SliderInteraction.slideThumb', (WidgetTester tester) async {
@@ -4023,6 +4072,7 @@ void main() {
       const Offset startOfTheSliderTrack = Offset(24, 300);
       const Offset centerOfTheSliderTrack = Offset(400, 300);
       const Offset endOfTheSliderTrack = Offset(800 - 24, 300);
+      final List<String> logs = <String>[];
 
       Widget buildApp() {
         return MaterialApp(
@@ -4033,11 +4083,18 @@ void main() {
                   value: value,
                   key: sliderKey,
                   allowedInteraction: SliderInteraction.slideThumb,
+                  onChangeStart: (double newValue) {
+                    logs.add('onChangeStart');
+                  },
                   onChanged: (double newValue) {
+                    logs.add('onChanged');
                     setState(() {
                       value = newValue;
                     });
                   },
+                  onChangeEnd: (double newValue) {
+                    logs.add('onChangeEnd');
+                  },
                 );
               }),
             ),
@@ -4047,17 +4104,21 @@ void main() {
 
       await tester.pumpWidget(buildApp());
 
+      expect(logs, isEmpty);
+
       // test tap
       final TestGesture gesture = await tester.startGesture(centerOfTheSliderTrack);
       await tester.pump();
       // has no effect, remains 1.0
       expect(value, 1.0);
+      expect(logs, isEmpty);
 
       // test slide
       await gesture.moveTo(startOfTheSliderTrack);
       await tester.pump();
       // has no effect, remains 1.0
       expect(value, 1.0);
+      expect(logs, isEmpty);
 
       // test slide thumb
       await gesture.up();
@@ -4065,22 +4126,36 @@ void main() {
       await tester.pump();
       // has no effect, remains 1.0
       expect(value, 1.0);
+      expect(logs, <String>['onChangeStart']);
+
       await gesture.moveTo(centerOfTheSliderTrack);
       await tester.pump();
       // changes from 1.0 -> 0.5
       expect(value, 0.5);
+      expect(logs, <String>['onChangeStart', 'onChanged']);
 
       // test tap inside overlay but not on thumb, then slide
       await gesture.up();
       // default overlay radius is 12, so 10 is inside the overlay
       await gesture.down(centerOfTheSliderTrack.translate(-10, 0));
       await tester.pump();
-      // has no effect, remains 1.0
+      // changes from 1.0 -> 0.5
       expect(value, 0.5);
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChangeEnd', 'onChangeStart']);
+
       await gesture.moveTo(endOfTheSliderTrack.translate(-10, 0));
       await tester.pump();
       // changes from 0.5 -> 1.0
       expect(value, 1.0);
+      expect(logs, <String>['onChangeStart', 'onChanged', 'onChangeEnd', 'onChangeStart', 'onChanged']);
+
+      await gesture.up();
+      await tester.pump();
+
+      expect(
+        logs,
+        <String>['onChangeStart', 'onChanged', 'onChangeEnd', 'onChangeStart', 'onChanged', 'onChangeEnd'],
+      );
     });
   });
 }