`Slider.onChangeStart` & `Slider.onChangeEnd` are not called on keyboard shortcuts (#126896)

fixes https://github.com/flutter/flutter/issues/123315
--------

This PR makes changes to the _actionHandler function used on the Slider.Dart Widget for Key Events. It ensures onChangeStart is called at the beginning of a Key Event and onChangeEnd at the end of one. This PR includes a test for the changes made.
I ran all existing tests after my changes were made and they passed.

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

@@ -684,6 +684,7 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
   void _actionHandler(_AdjustSliderIntent intent) {
     final _RenderSlider renderSlider = _renderObjectKey.currentContext!.findRenderObject()! as _RenderSlider;
     final TextDirection textDirection = Directionality.of(_renderObjectKey.currentContext!);
+
     switch (intent.type) {
       case _SliderAdjustmentType.right:
         switch (textDirection) {
@@ -1802,15 +1803,33 @@ class _RenderSlider extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
 
   void increaseAction() {
     if (isInteractive) {
-      onChanged!(clampDouble(value + _semanticActionUnit, 0.0, 1.0));
+      onChangeStart!(currentValue);
+      final double increase = increaseValue();
+      onChanged!(increase);
+      onChangeEnd!(increase);
     }
   }
 
   void decreaseAction() {
     if (isInteractive) {
-      onChanged!(clampDouble(value - _semanticActionUnit, 0.0, 1.0));
+      onChangeStart!(currentValue);
+      final double decrease = decreaseValue();
+      onChanged!(decrease);
+      onChangeEnd!(decrease);
     }
   }
+
+  double get currentValue {
+    return clampDouble(value, 0.0, 1.0);
+  }
+
+  double increaseValue() {
+    return clampDouble(value + _semanticActionUnit, 0.0, 1.0);
+  }
+
+  double decreaseValue() {
+    return clampDouble(value - _semanticActionUnit, 0.0, 1.0);
+  }
 }
 
 class _AdjustSliderIntent extends Intent {

packages/flutter/test/material/slider_test.dart

@@ -2125,17 +2125,29 @@ void main() {
 
   testWidgets('Slider can be incremented and decremented by keyboard shortcuts - LTR', (WidgetTester tester) async {
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
-    double value = 0.5;
+    double startValue = 0.0;
+    double currentValue = 0.5;
+    double endValue = 0.0;
     await tester.pumpWidget(
       MaterialApp(
         home: Material(
           child: Center(
             child: StatefulBuilder(builder: (BuildContext context, StateSetter setState) {
               return Slider(
-                value: value,
+                value: currentValue,
+                onChangeStart: (double newValue) {
+                  setState(() {
+                    startValue = newValue;
+                  });
+                },
                 onChanged: (double newValue) {
                   setState(() {
-                    value = newValue;
+                    currentValue = newValue;
+                  });
+                },
+                onChangeEnd: (double newValue) {
+                  setState(() {
+                    endValue = newValue;
                   });
                 },
                 autofocus: true,
@@ -2149,34 +2161,54 @@ void main() {
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowRight);
     await tester.pumpAndSettle();
-    expect(value, 0.55);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.55);
+    expect(endValue, 0.55);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowLeft);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.55);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
     await tester.pumpAndSettle();
-    expect(value, 0.55);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.55);
+    expect(endValue, 0.55);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.55);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.android,  TargetPlatform.fuchsia, TargetPlatform.linux, TargetPlatform.windows }));
 
   testWidgets('Slider can be incremented and decremented by keyboard shortcuts - LTR', (WidgetTester tester) async {
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
-    double value = 0.5;
+    double startValue = 0.0;
+    double currentValue = 0.5;
+    double endValue = 0.0;
     await tester.pumpWidget(
       MaterialApp(
         home: Material(
           child: Center(
             child: StatefulBuilder(builder: (BuildContext context, StateSetter setState) {
               return Slider(
-                value: value,
+                value: currentValue,
+                onChangeStart: (double newValue) {
+                  setState(() {
+                    startValue = newValue;
+                  });
+                },
                 onChanged: (double newValue) {
                   setState(() {
-                    value = newValue;
+                    currentValue = newValue;
+                  });
+                },
+                onChangeEnd: (double newValue) {
+                  setState(() {
+                    endValue = newValue;
                   });
                 },
                 autofocus: true,
@@ -2190,24 +2222,34 @@ void main() {
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowRight);
     await tester.pumpAndSettle();
-    expect(value, 0.6);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.6);
+    expect(endValue, 0.6);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowLeft);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.6);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
     await tester.pumpAndSettle();
-    expect(value, 0.6);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.6);
+    expect(endValue, 0.6);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.6);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
   testWidgets('Slider can be incremented and decremented by keyboard shortcuts - RTL', (WidgetTester tester) async {
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
-    double value = 0.5;
+    double startValue = 0.0;
+    double currentValue = 0.5;
+    double endValue = 0.0;
     await tester.pumpWidget(
       MaterialApp(
         home: Material(
@@ -2216,10 +2258,20 @@ void main() {
               return Directionality(
                 textDirection: TextDirection.rtl,
                 child: Slider(
-                  value: value,
+                  value: currentValue,
+                  onChangeStart: (double newValue) {
+                    setState(() {
+                      startValue = newValue;
+                    });
+                  },
                   onChanged: (double newValue) {
                     setState(() {
-                      value = newValue;
+                      currentValue = newValue;
+                    });
+                  },
+                  onChangeEnd: (double newValue) {
+                    setState(() {
+                      endValue = newValue;
                     });
                   },
                   autofocus: true,
@@ -2234,24 +2286,34 @@ void main() {
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowRight);
     await tester.pumpAndSettle();
-    expect(value, 0.45);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.45);
+    expect(endValue, 0.45);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowLeft);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.45);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
     await tester.pumpAndSettle();
-    expect(value, 0.55);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.55);
+    expect(endValue, 0.55);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.55);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.android,  TargetPlatform.fuchsia, TargetPlatform.linux, TargetPlatform.windows }));
 
   testWidgets('Slider can be incremented and decremented by keyboard shortcuts - RTL', (WidgetTester tester) async {
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
-    double value = 0.5;
+    double startValue = 0.0;
+    double currentValue = 0.5;
+    double endValue = 0.0;
     await tester.pumpWidget(
       MaterialApp(
         home: Material(
@@ -2260,10 +2322,20 @@ void main() {
               return Directionality(
                 textDirection: TextDirection.rtl,
                 child: Slider(
-                  value: value,
+                  value: currentValue,
+                  onChangeStart: (double newValue) {
+                    setState(() {
+                      startValue = newValue;
+                    });
+                  },
                   onChanged: (double newValue) {
                     setState(() {
-                      value = newValue;
+                      currentValue = newValue;
+                    });
+                  },
+                  onChangeEnd: (double newValue) {
+                    setState(() {
+                      endValue = newValue;
                     });
                   },
                   autofocus: true,
@@ -2278,19 +2350,27 @@ void main() {
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowRight);
     await tester.pumpAndSettle();
-    expect(value, 0.4);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.4);
+    expect(endValue, 0.4);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowLeft);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.4);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
     await tester.pumpAndSettle();
-    expect(value, 0.6);
+    expect(startValue, 0.5);
+    expect(currentValue, 0.6);
+    expect(endValue, 0.6);
 
     await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
     await tester.pumpAndSettle();
-    expect(value, 0.5);
+    expect(startValue, 0.6);
+    expect(currentValue, 0.5);
+    expect(endValue, 0.5);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
   testWidgets('In directional nav, Slider can be navigated out of by using up and down arrows', (WidgetTester tester) async {