fix: Inconsistency of SelectionArea when scrolling (#128765)

This PR fixes inconsistency of SelectionArea when scrolling by removing the `_clearSelection` on `longPressUpdate` gesture. This in turn makes the selection UX much better.

*List which issues are fixed by this PR. You must list at least one issue.*
Fixes: https://github.com/flutter/flutter/issues/120892

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

@@ -439,8 +439,7 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
         instance
           ..onLongPressStart = _handleTouchLongPressStart
           ..onLongPressMoveUpdate = _handleTouchLongPressMoveUpdate
-          ..onLongPressEnd = _handleTouchLongPressEnd
-          ..onLongPressCancel = _clearSelection;
+          ..onLongPressEnd = _handleTouchLongPressEnd;
       },
     );
   }

packages/flutter/test/widgets/selectable_region_test.dart

@@ -310,6 +310,82 @@ void main() {
       expect(edgeEvent.globalPosition, const Offset(200.0, 50.0));
     });
 
+  testWidgets(
+    'touch long press cancel does not send ClearSelectionEvent',
+    (WidgetTester tester) async {
+      final UniqueKey spy = UniqueKey();
+      await tester.pumpWidget(
+          MaterialApp(
+            home: SelectableRegion(
+              focusNode: FocusNode(),
+              selectionControls: materialTextSelectionControls,
+              child: SelectionSpy(key: spy),
+            ),
+          ),
+      );
+      await tester.pumpAndSettle();
+
+      final RenderSelectionSpy renderSelectionSpy =
+          tester.renderObject<RenderSelectionSpy>(find.byKey(spy));
+      renderSelectionSpy.events.clear();
+      final TestGesture gesture =
+          await tester.startGesture(const Offset(200.0, 200.0));
+
+      addTearDown(gesture.removePointer);
+
+      await tester.pump(const Duration(milliseconds: 500));
+      await gesture.cancel();
+      expect(
+        renderSelectionSpy.events.any((SelectionEvent element) => element is ClearSelectionEvent),
+        isFalse,
+      );
+    },
+  );
+
+    testWidgets(
+      'scrolling after the selection does not send ClearSelectionEvent',
+      (WidgetTester tester) async {
+        // Regression test for https://github.com/flutter/flutter/issues/128765
+        final UniqueKey spy = UniqueKey();
+        await tester.pumpWidget(
+          MaterialApp(
+            home: SizedBox(
+              height: 750,
+              child: SingleChildScrollView(
+                child: SizedBox(
+                  height: 2000,
+                  child: SelectableRegion(
+                    focusNode: FocusNode(),
+                    selectionControls: materialTextSelectionControls,
+                    child: SelectionSpy(key: spy),
+                  ),
+                ),
+              ),
+            ),
+          ),
+        );
+        await tester.pumpAndSettle();
+
+        final RenderSelectionSpy renderSelectionSpy = tester.renderObject<RenderSelectionSpy>(find.byKey(spy));
+        renderSelectionSpy.events.clear();
+        final TestGesture selectGesture = await tester.startGesture(const Offset(200.0, 200.0));
+        addTearDown(selectGesture.removePointer);
+        await tester.pump(const Duration(milliseconds: 500));
+        await selectGesture.up();
+        expect(renderSelectionSpy.events.length, 1);
+        expect(renderSelectionSpy.events[0], isA<SelectWordSelectionEvent>());
+
+        renderSelectionSpy.events.clear();
+         final TestGesture scrollGesture =
+            await tester.startGesture(const Offset(250.0, 850.0));
+        await tester.pump(const Duration(milliseconds: 500));
+        await scrollGesture.moveTo(Offset.zero);
+        await scrollGesture.up();
+        await tester.pumpAndSettle();
+        expect(renderSelectionSpy.events.length, 0);
+      },
+    );
+
     testWidgets('mouse long press does not send select-word event', (WidgetTester tester) async {
       final UniqueKey spy = UniqueKey();
       await tester.pumpWidget(