Update alwaysNeedsCompositing in RenderParagraph (flutter#135076)

fix flutter#111370

According to 

https://github.com/flutter/flutter/blob/5b47fef613e2d74ae44e1e22c2a3495294db180e/packages/flutter/lib/src/rendering/box.dart#L1259
: A [RenderBox] that uses methods on [PaintingContext] that introduce
new
/// layers should override the [alwaysNeedsCompositing] getter and set
it to
/// true.

set [alwaysNeedsCompositing] to true when RenderParagraph introduces
LeaderLayer for selection handles.



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above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
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[test-exempt].
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Author: hannah-hyj

packages/flutter/lib/src/rendering/paragraph.dart

@@ -388,6 +388,9 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
     }
     _lastSelectableFragments ??= _getSelectableFragments();
     _lastSelectableFragments!.forEach(_registrar!.add);
+    if (_lastSelectableFragments!.isNotEmpty) {
+      markNeedsCompositingBitsUpdate();
+    }
   }
 
   void _removeSelectionRegistrarSubscription() {
@@ -425,6 +428,9 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
     _lastSelectableFragments = null;
   }
 
+  @override
+  bool get alwaysNeedsCompositing => _lastSelectableFragments?.isNotEmpty ?? false;
+
   @override
   void markNeedsLayout() {
     _lastSelectableFragments?.forEach((_SelectableFragment element) => element.didChangeParagraphLayout());

packages/flutter/test/material/selection_area_test.dart

@@ -70,6 +70,63 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/111370
+  testWidgetsWithLeakTracking('Handle is correctly transformed when the text is inside of a FittedBox ',(WidgetTester tester) async {
+      final Key textKey = UniqueKey();
+      await tester.pumpWidget(
+        MaterialApp(
+          color: const Color(0xFF2196F3),
+          home: Scaffold(
+            body: SelectionArea(
+              child: SizedBox(
+                height: 100,
+                child: FittedBox(
+                  fit: BoxFit.fill,
+                  child: Text('test', key: textKey),
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      final TestGesture longpress = await tester.startGesture(const Offset(10, 10));
+      addTearDown(longpress.removePointer);
+      await tester.pump(const Duration(milliseconds: 500));
+      await longpress.up();
+
+      // Text box is scaled by 5.
+      final RenderBox textBox = tester.firstRenderObject(find.byKey(textKey));
+      expect(textBox.size.height, 20.0);
+      final Offset textPoint = textBox.localToGlobal(const Offset(0, 20));
+      expect(textPoint, equals(const Offset(0, 100)));
+
+      // Find handles and verify their sizes.
+      expect(find.byType(Overlay), findsOneWidget);
+      expect(find.descendant(of: find.byType(Overlay),matching: find.byType(CustomPaint),),findsNWidgets(2));
+      final Iterable<RenderBox> handles = tester.renderObjectList(find.descendant(
+        of: find.byType(Overlay),
+        matching: find.byType(CustomPaint),
+      ));
+
+      // The handle height is determined by the formula:
+      // textLineHeight + _kSelectionHandleRadius * 2 - _kSelectionHandleOverlap .
+      // The text line height will be the value of the fontSize.
+      // The constant _kSelectionHandleRadius has the value of 6.
+      // The constant _kSelectionHandleOverlap has the value of 1.5.
+      // The handle height before scaling is 20.0 + 6 * 2 - 1.5 = 30.5.
+
+      final double handleHeightBeforeScaling = handles.first.size.height;
+      expect(handleHeightBeforeScaling, 30.5);
+
+      final Offset handleHeightAfterScaling = handles.first.localToGlobal(const Offset(0, 30.5)) - handles.first.localToGlobal(Offset.zero);
+
+      // The handle height after scaling is  30.5 * 5 = 152.5
+      expect(handleHeightAfterScaling, equals(const Offset(0.0, 152.5)));
+    },
+    skip: isBrowser, // [intended]
+    variant: const TargetPlatformVariant(<TargetPlatform>{TargetPlatform.iOS}),
+  );
 
   testWidgetsWithLeakTracking('builds the default context menu by default', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode();