[web] When a canvas element is reused and is first element in child list, preserve zIndex.

lib/web_ui/lib/src/engine/canvas_pool.dart

@@ -80,12 +80,10 @@ class _CanvasPool extends _SaveStackTracking {
 
   void _createCanvas() {
     bool requiresClearRect = false;
+    bool reused = false;
     if (_reusablePool != null && _reusablePool.isNotEmpty) {
       _canvas = _reusablePool.removeAt(0);
-      // If a canvas is the first element we set z-index = -1 to workaround
-      // blink compositing bug. To make sure this does not leak when reused
-      // reset z-index.
-      _canvas.style.removeProperty('z-index');
+      reused = true;
       requiresClearRect = true;
     } else {
       // Compute the final CSS canvas size given the actual pixel count we
@@ -116,6 +114,12 @@ class _CanvasPool extends _SaveStackTracking {
         ..height = '${cssHeight}px';
     }
 
+    // Before appending canvas, check if canvas is already on rootElement. This
+    // optimization prevents DOM .append call when a PersistentSurface is
+    // reused. Reading lastChild is faster than append call.
+    if (_rootElement.lastChild != _canvas) {
+      _rootElement.append(_canvas);
+    }
     // When the picture has a 90-degree transform and clip in its
     // ancestor layers, it triggers a bug in Blink and Webkit browsers
     // that results in canvas obscuring text that should be painted on
@@ -126,16 +130,15 @@ class _CanvasPool extends _SaveStackTracking {
     // Possible Blink bugs that are causing this:
     // * https://bugs.chromium.org/p/chromium/issues/detail?id=370604
     // * https://bugs.chromium.org/p/chromium/issues/detail?id=586601
-    final bool isFirstChildElement = _rootElement.firstChild == null;
-    if (isFirstChildElement) {
+    if (_rootElement.firstChild == _canvas) {
       _canvas.style.zIndex = '-1';
+    } else if (reused) {
+      // If a canvas is the first element we set z-index = -1 to workaround
+      // blink compositing bug. To make sure this does not leak when reused
+      // reset z-index.
+      _canvas.style.removeProperty('z-index');
     }
-    // Before appending canvas, check if canvas is already on rootElement. This
-    // optimization prevents DOM .append call when a PersistentSurface is
-    // reused. Reading lastChild is faster than append call.
-    if (_rootElement.lastChild != _canvas) {
-      _rootElement.append(_canvas);
-    }
+
     _context = _canvas.context2D;
     _contextHandle = ContextStateHandle(_context);
     _initializeViewport(requiresClearRect);

lib/web_ui/test/compositing_test.dart

@@ -177,6 +177,39 @@ void main() {
     }, // TODO(nurhan): https://github.com/flutter/flutter/issues/46638
         skip: (browserEngine == BrowserEngine.firefox));
   });
+
+  group('Compositing order', () {
+    // Regression test for https://github.com/flutter/flutter/issues/55058
+    //
+    // When BitmapCanvas uses multiple elements to paint, the very first
+    // canvas needs to have a -1 zIndex so it can preserve compositing order.
+    test('First canvas element should retain -1 zIndex after update', () async {
+      final SurfaceSceneBuilder builder = SurfaceSceneBuilder();
+      final Picture picture1 = _drawPicture();
+      EngineLayer oldLayer = builder.pushClipRect(
+        const Rect.fromLTRB(10, 10, 300, 300),
+      );
+      builder.addPicture(Offset.zero, picture1);
+      builder.pop();
+
+      html.HtmlElement content = builder.build().webOnlyRootElement;
+      expect(content.querySelector('canvas').style.zIndex, '-1');
+
+      // Force update to scene which will utilize reuse code path.
+      final SurfaceSceneBuilder builder2 = SurfaceSceneBuilder();
+      builder2.pushClipRect(
+          const Rect.fromLTRB(5, 10, 300, 300),
+          oldLayer: oldLayer
+      );
+      final Picture picture2 = _drawPicture();
+      builder2.addPicture(Offset.zero, picture2);
+      builder2.pop();
+
+      html.HtmlElement contentAfterReuse = builder2.build().webOnlyRootElement;
+      expect(contentAfterReuse.querySelector('canvas').style.zIndex, '-1');
+    });
+  });
+
 }
 
 typedef TestLayerBuilder = EngineLayer Function(
@@ -312,3 +345,32 @@ class MockPersistedPicture extends PersistedPicture {
   @override
   int get bitmapPixelCount => 0;
 }
+
+Picture _drawPicture() {
+  const double offsetX = 50;
+  const double offsetY = 50;
+  final EnginePictureRecorder recorder = PictureRecorder();
+  final RecordingCanvas canvas =
+  recorder.beginRecording(const Rect.fromLTRB(0, 0, 400, 400));
+  canvas.drawCircle(
+      Offset(offsetX + 10, offsetY + 10), 10, Paint()..style = PaintingStyle.fill);
+  canvas.drawCircle(
+      Offset(offsetX + 60, offsetY + 10),
+      10,
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = const Color.fromRGBO(255, 0, 0, 1));
+  canvas.drawCircle(
+      Offset(offsetX + 10, offsetY + 60),
+      10,
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = const Color.fromRGBO(0, 255, 0, 1));
+  canvas.drawCircle(
+      Offset(offsetX + 60, offsetY + 60),
+      10,
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = const Color.fromRGBO(0, 0, 255, 1));
+  return recorder.endRecording();
+}