Cover text_selection tests with leak tracking. (#137009)

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

@@ -364,7 +364,8 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
        _readOnly = readOnly,
        _forceLine = forceLine,
        _clipBehavior = clipBehavior,
-       _hasFocus = hasFocus ?? false {
+       _hasFocus = hasFocus ?? false,
+       _disposeShowCursor = showCursor == null {
     assert(!_showCursor.value || cursorColor != null);
 
     _selectionPainter.highlightColor = selectionColor;
@@ -405,6 +406,10 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     _selectionPainter.dispose();
     _caretPainter.dispose();
     _textPainter.dispose();
+    if (_disposeShowCursor) {
+      _showCursor.dispose();
+      _disposeShowCursor = false;
+    }
     super.dispose();
   }
 
@@ -879,6 +884,8 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     _caretPainter.backgroundCursorColor = value;
   }
 
+  bool _disposeShowCursor;
+
   /// Whether to paint the cursor.
   ValueNotifier<bool> get showCursor => _showCursor;
   ValueNotifier<bool> _showCursor;
@@ -889,6 +896,10 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     if (attached) {
       _showCursor.removeListener(_showHideCursor);
     }
+    if (_disposeShowCursor) {
+      _showCursor.dispose();
+      _disposeShowCursor = false;
+    }
     _showCursor = value;
     if (attached) {
       _showHideCursor();

packages/flutter/test/widgets/text_selection_toolbar_layout_delegate_test.dart

@@ -4,9 +4,10 @@
 
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 void main() {
-  testWidgets('positions itself at anchorAbove if it fits', (WidgetTester tester) async {
+  testWidgetsWithLeakTracking('positions itself at anchorAbove if it fits', (WidgetTester tester) async {
     late StateSetter setState;
     const double height = 43.0;
     const double anchorBelowY = 500.0;