[web] do not allocate canvases just for text

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

@@ -76,13 +76,6 @@ class CanvasPool extends _SaveStackTracking {
     translate(transform.dx, transform.dy);
   }
 
-  /// Returns true if no canvas has been allocated yet.
-  bool get isEmpty => _canvas == null;
-
-  /// Returns true if a canvas has been allocated for use.
-  bool get isNotEmpty => _canvas != null;
-
-
   /// Returns [CanvasRenderingContext2D] api to draw into this canvas.
   html.CanvasRenderingContext2D get context {
     html.CanvasRenderingContext2D? ctx = _context;
@@ -106,12 +99,28 @@ class CanvasPool extends _SaveStackTracking {
     return _contextHandle!;
   }
 
-  /// Prevents active canvas to be used for rendering and prepares a new
-  /// canvas allocation on next drawing request that will require one.
+  /// Returns true if a canvas is currently available for drawing.
+  ///
+  /// Calling [contextHandle] or, transitively, any of the `draw*` methods while
+  /// this returns true will reuse the existing canvas. Otherwise, a new canvas
+  /// will be allocated.
+  ///
+  /// Previously allocated and closed canvases (see [closeCanvas]) are not
+  /// considered by this getter.
+  bool get hasCanvas => _canvas != null;
+
+  /// Stops the currently available canvas from receiving any further drawing
+  /// commands.
+  ///
+  /// After calling this method, a subsequent call to [contextHandle] or,
+  /// transitively, any of the `draw*` methods will cause a new canvas to be
+  /// allocated.
   ///
-  /// Saves current canvas so we can dispose
-  /// and replay the clip/transform stack on top of new canvas.
-  void closeCurrentCanvas() {
+  /// The closed canvas becomes an "active" canvas, that is a canvas that's used
+  /// to render picture content in the current frame. Active canvases may be
+  /// reused in other pictures if their contents are no longer needed for this
+  /// picture.
+  void closeCanvas() {
     assert(_rootElement != null);
     // Place clean copy of current canvas with context stack restored and paint
     // reset into pool.

lib/web_ui/lib/src/engine/html/bitmap_canvas.dart

@@ -370,7 +370,7 @@ class BitmapCanvas extends EngineCanvas {
       _renderStrategy.isInsideSvgFilterTree ||
       (_preserveImageData == false && _contains3dTransform) ||
       (_childOverdraw &&
-          _canvasPool.isEmpty &&
+          !_canvasPool.hasCanvas &&
           paint.maskFilter == null &&
           paint.shader == null &&
           paint.style != ui.PaintingStyle.stroke);
@@ -384,7 +384,7 @@ class BitmapCanvas extends EngineCanvas {
       ((_childOverdraw ||
               _renderStrategy.hasImageElements ||
               _renderStrategy.hasParagraphs) &&
-          _canvasPool.isEmpty &&
+          !_canvasPool.hasCanvas &&
           paint.maskFilter == null &&
           paint.shader == null);
 
@@ -469,7 +469,7 @@ class BitmapCanvas extends EngineCanvas {
       element.style.mixBlendMode = blendModeToCssMixBlendMode(blendMode) ?? '';
     }
     // Switch to preferring DOM from now on, and close the current canvas.
-    _closeCurrentCanvas();
+    _closeCanvas();
   }
 
   @override
@@ -626,7 +626,7 @@ class BitmapCanvas extends EngineCanvas {
       _applyTargetSize(
           imageElement, image.width.toDouble(), image.height.toDouble());
     }
-    _closeCurrentCanvas();
+    _closeCanvas();
   }
 
   html.ImageElement _reuseOrCreateImage(HtmlImage htmlImage) {
@@ -770,7 +770,7 @@ class BitmapCanvas extends EngineCanvas {
         restore();
       }
     }
-    _closeCurrentCanvas();
+    _closeCanvas();
   }
 
   void _applyTargetSize(
@@ -882,8 +882,8 @@ class BitmapCanvas extends EngineCanvas {
   //   |--- <img>
   // Any drawing operations after these tags should allocate a new canvas,
   // instead of drawing into earlier canvas.
-  void _closeCurrentCanvas() {
-    _canvasPool.closeCurrentCanvas();
+  void _closeCanvas() {
+    _canvasPool.closeCanvas();
     _childOverdraw = true;
     _cachedLastCssFont = null;
   }
@@ -939,16 +939,24 @@ class BitmapCanvas extends EngineCanvas {
   void drawParagraph(CanvasParagraph paragraph, ui.Offset offset) {
     assert(paragraph.isLaidOut);
 
-    /// - paragraph.drawOnCanvas checks that the text styling doesn't include
-    /// features that prevent text from being rendered correctly using canvas.
-    /// - _childOverdraw check prevents sandwitching multiple canvas elements
-    /// when we have alternating paragraphs and other drawing commands that are
-    /// suitable for canvas.
-    /// - To make sure an svg filter is applied correctly to paragraph we
-    /// check isInsideSvgFilterTree to make sure dom node doesn't have any
-    /// parents that apply one.
-    if (paragraph.drawOnCanvas && _childOverdraw == false &&
-        !_renderStrategy.isInsideSvgFilterTree) {
+    // Normally, text is composited as a plain HTML <p> tag. However, if a
+    // bitmap canvas was used for a preceding drawing command, then it's more
+    // efficient to continue compositing into the existing canvas, if possible.
+    // Whether it's possible to composite a paragraph into a 2D canvas depends
+    // on the following:
+    final bool canCompositeIntoBitmapCanvas =
+        // Cannot composite if the paragraph cannot be drawn into bitmap canvas
+        // in the first place.
+        paragraph.canDrawOnCanvas &&
+        // Cannot composite if there's no bitmap canvas to composite into.
+        // Creating a new bitmap canvas just to draw text doesn't make sense.
+        _canvasPool.hasCanvas &&
+        !_childOverdraw &&
+        // Bitmap canvas introduces correctness issues in the presence of SVG
+        // filters, so prefer plain HTML in this case.
+        !_renderStrategy.isInsideSvgFilterTree;
+
+    if (canCompositeIntoBitmapCanvas) {
       paragraph.paint(this, offset);
       return;
     }
@@ -977,7 +985,7 @@ class BitmapCanvas extends EngineCanvas {
     paragraphElement.style
       ..left = '0px'
       ..top = '0px';
-    _closeCurrentCanvas();
+    _closeCanvas();
   }
 
   /// Draws vertices on a gl context.

lib/web_ui/lib/src/engine/text/canvas_paragraph.dart

@@ -31,7 +31,7 @@ class CanvasParagraph implements ui.Paragraph {
     required this.paragraphStyle,
     required this.plainText,
     required this.placeholderCount,
-    required this.drawOnCanvas,
+    required this.canDrawOnCanvas,
   });
 
   /// The flat list of spans that make up this paragraph.
@@ -47,7 +47,10 @@ class CanvasParagraph implements ui.Paragraph {
   final int placeholderCount;
 
   /// Whether this paragraph can be drawn on a bitmap canvas.
-  final bool drawOnCanvas;
+  ///
+  /// Some text features cannot be rendered into a 2D canvas and must use HTML,
+  /// such as font features and text decorations.
+  final bool canDrawOnCanvas;
 
   @override
   double get width => _layoutService.width;
@@ -623,7 +626,7 @@ class CanvasParagraphBuilder implements ui.ParagraphBuilder {
     }
   }
 
-  bool _drawOnCanvas = true;
+  bool _canDrawOnCanvas = true;
 
   @override
   void addText(String text) {
@@ -632,24 +635,24 @@ class CanvasParagraphBuilder implements ui.ParagraphBuilder {
     _plainTextBuffer.write(text);
     final int end = _plainTextBuffer.length;
 
-    if (_drawOnCanvas) {
+    if (_canDrawOnCanvas) {
       final ui.TextDecoration? decoration = style.decoration;
       if (decoration != null && decoration != ui.TextDecoration.none) {
-        _drawOnCanvas = false;
+        _canDrawOnCanvas = false;
       }
     }
 
-    if (_drawOnCanvas) {
+    if (_canDrawOnCanvas) {
       final List<ui.FontFeature>? fontFeatures = style.fontFeatures;
       if (fontFeatures != null && fontFeatures.isNotEmpty) {
-        _drawOnCanvas = false;
+        _canDrawOnCanvas = false;
       }
     }
 
-    if (_drawOnCanvas) {
+    if (_canDrawOnCanvas) {
       final List<ui.FontVariation>? fontVariations = style.fontVariations;
       if (fontVariations != null && fontVariations.isNotEmpty) {
-        _drawOnCanvas = false;
+        _canDrawOnCanvas = false;
       }
     }
 
@@ -663,7 +666,7 @@ class CanvasParagraphBuilder implements ui.ParagraphBuilder {
       paragraphStyle: _paragraphStyle,
       plainText: _plainTextBuffer.toString(),
       placeholderCount: _placeholderCount,
-      drawOnCanvas: _drawOnCanvas,
+      canDrawOnCanvas: _canDrawOnCanvas,
     );
   }
 }

lib/web_ui/test/html/bitmap_canvas_golden_test.dart

@@ -270,4 +270,26 @@ Future<void> testMain() async {
       pixelComparison: PixelComparison.precise,
     );
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/96498. When
+  // a picture is made of just text that can be rendered using plain HTML,
+  // BitmapCanvas should not create any <canvas> elements as they are expensive.
+  test('does not allocate bitmap canvas just for text', () async {
+    canvas = BitmapCanvas(const Rect.fromLTWH(0, 0, 50, 50), RenderStrategy());
+
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(fontFamily: 'Roboto'));
+    builder.addText('Hello');
+    final CanvasParagraph paragraph = builder.build() as CanvasParagraph;
+    paragraph.layout(const ParagraphConstraints(width: 1000));
+
+    canvas.drawParagraph(paragraph, const Offset(8.5, 8.5));
+    expect(
+      canvas.rootElement.querySelectorAll('canvas'),
+      isEmpty,
+    );
+    expect(
+      canvas.rootElement.querySelectorAll('flt-paragraph').single.innerText,
+      'Hello',
+    );
+  });
 }

lib/web_ui/test/html/compositing/compositing_golden_test.dart

@@ -847,7 +847,7 @@ void _testCullRectComputation() {
         final RecordingCanvas canvas = recorder.beginRecording(outerClip);
         canvas.drawParagraph(paragraph, const ui.Offset(8.5, 8.5));
         final ui.Picture picture = recorder.endRecording();
-        expect(paragraph.drawOnCanvas, isFalse);
+        expect(paragraph.canDrawOnCanvas, isFalse);
 
         builder.addPicture(
           ui.Offset.zero,
@@ -861,7 +861,7 @@ void _testCullRectComputation() {
         final RecordingCanvas canvas = recorder.beginRecording(innerClip);
         canvas.drawParagraph(paragraph, ui.Offset(8.5, 8.5 + innerClip.top));
         final ui.Picture picture = recorder.endRecording();
-        expect(paragraph.drawOnCanvas, isFalse);
+        expect(paragraph.canDrawOnCanvas, isFalse);
 
         builder.addPicture(
           ui.Offset.zero,