Set stable color for semantics debugger (#157884)

fixes https://github.com/flutter/flutter/issues/156242



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packages/flutter/lib/src/semantics/semantics.dart

@@ -2092,8 +2092,7 @@ class SemanticsNode with DiagnosticableTreeMixin {
   /// Visits the immediate children of this node.
   ///
   /// This function calls visitor for each immediate child until visitor returns
-  /// false. Returns true if all the visitor calls returned true, otherwise
-  /// returns false.
+  /// false.
   void visitChildren(SemanticsNodeVisitor visitor) {
     if (_children != null) {
       for (final SemanticsNode child in _children!) {

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

@@ -202,7 +202,7 @@ class _SemanticsDebuggerPainter extends CustomPainter {
     canvas.save();
     canvas.scale(1.0 / devicePixelRatio, 1.0 / devicePixelRatio);
     if (rootNode != null) {
-      _paint(canvas, rootNode, _findDepth(rootNode));
+      _paint(canvas, rootNode, _findDepth(rootNode), 0, 0);
     }
     if (pointerPosition != null) {
       final Paint paint = Paint();
@@ -332,14 +332,14 @@ class _SemanticsDebuggerPainter extends CustomPainter {
     return childrenDepth + 1;
   }
 
-  void _paint(Canvas canvas, SemanticsNode node, int rank) {
+  void _paint(Canvas canvas, SemanticsNode node, int rank, int indexInParent, int level) {
     canvas.save();
     if (node.transform != null) {
       canvas.transform(node.transform!.storage);
     }
     final Rect rect = node.rect;
     if (!rect.isEmpty) {
-      final Color lineColor = Color(0xFF000000 + math.Random(node.id).nextInt(0xFFFFFF));
+      final Color lineColor = _colorForNode(indexInParent, level);
       final Rect innerRect = rect.deflate(rank * 1.0);
       if (innerRect.isEmpty) {
         final Paint fill = Paint()
@@ -361,13 +361,31 @@ class _SemanticsDebuggerPainter extends CustomPainter {
     }
     if (!node.mergeAllDescendantsIntoThisNode) {
       final int childRank = rank - 1;
+      final int childLevel = level + 1;
+      int childIndex = 0;
       node.visitChildren((SemanticsNode child) {
-        _paint(canvas, child, childRank);
+        _paint(canvas, child, childRank, childIndex, childLevel);
+        childIndex += 1;
         return true;
       });
     }
     canvas.restore();
   }
+
+  static Color _colorForNode(int index, int level) {
+    return HSLColor.fromAHSL(
+      1.0,
+      // Use custom hash to ensure stable value regardless of Dart changes
+      360.0 * math.Random(_getColorSeed(index, level)).nextDouble(),
+      1.0,
+      0.7,
+    ).toColor();
+  }
+
+  static int _getColorSeed(int level, int index) {
+    // Should be no collision as long as children number < 10000.
+    return level * 10000 + index;
+  }
 }
 
 /// A widget ignores pointer event but still keeps semantics actions.

packages/flutter/test/widgets/semantics_debugger_test.dart

@@ -61,6 +61,26 @@ void main() {
     expect(true, isTrue); // expect that we reach here without crashing
   });
 
+  testWidgets('SemanticsDebugger draw persistent color based on structure', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: SemanticsDebugger(
+          child: Stack(
+            children: <Widget>[
+              Semantics(
+                container: true,
+                child: Semantics(container: true),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(SemanticsDebugger), paints..rect()..rect(color: const Color(0xFFF866FF)));
+  });
+
   testWidgets('SemanticsDebugger reparents subtree', (WidgetTester tester) async {
     final GlobalKey key = GlobalKey();