fix canvas drawLine bugs (flutter#38753)

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

@@ -538,20 +538,6 @@ class BitmapCanvas extends EngineCanvas {
     if (_useDomForRenderingFill(paint)) {
       final Matrix4 transform = _canvasPool.currentTransform;
       final SurfacePath surfacePath = path as SurfacePath;
-      final ui.Rect? pathAsLine = surfacePath.toStraightLine();
-      if (pathAsLine != null) {
-        ui.Rect rect = (pathAsLine.top == pathAsLine.bottom)
-            ? ui.Rect.fromLTWH(
-                pathAsLine.left, pathAsLine.top, pathAsLine.width, 1)
-            : ui.Rect.fromLTWH(
-                pathAsLine.left, pathAsLine.top, 1, pathAsLine.height);
-
-        rect = adjustRectForDom(rect, paint);
-        final DomHTMLElement element = buildDrawRectElement(
-            rect, paint, 'draw-rect', _canvasPool.currentTransform);
-        _drawElement(element, rect.topLeft, paint);
-        return;
-      }
 
       final ui.Rect? pathAsRect = surfacePath.toRect();
       if (pathAsRect != null) {
@@ -563,16 +549,15 @@ class BitmapCanvas extends EngineCanvas {
         drawRRect(pathAsRRect, paint);
         return;
       }
-      final ui.Rect pathBounds = surfacePath.getBounds();
-      final DomElement svgElm = pathToSvgElement(
-          surfacePath, paint, '${pathBounds.right}', '${pathBounds.bottom}');
+      final DomElement svgElm = pathToSvgElement(surfacePath, paint);
       if (!_canvasPool.isClipped) {
         final DomCSSStyleDeclaration style = svgElm.style;
         style.position = 'absolute';
         if (!transform.isIdentity()) {
           style
             ..transform = matrix4ToCssTransform(transform)
-            ..transformOrigin = '0 0 0';
+            ..transformOrigin = '0 0 0'
+            ..overflow = 'visible';
         }
       }
       _applyFilter(svgElm, paint);

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

@@ -392,9 +392,7 @@ class PersistedPhysicalShape extends PersistedContainerSurface
         path,
         SurfacePaintData()
           ..style = ui.PaintingStyle.fill
-          ..color = color.value,
-        '${pathBounds2.right}',
-        '${pathBounds2.bottom}');
+          ..color = color.value);
 
     /// Render element behind the clipped content.
     rootElement!.insertBefore(_svgElement!, childContainer);

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

@@ -14,6 +14,7 @@ import '../svg.dart';
 import '../text/canvas_paragraph.dart';
 import '../util.dart';
 import '../vector_math.dart';
+import 'bitmap_canvas.dart';
 import 'painting.dart';
 import 'path/path.dart';
 import 'path/path_to_svg.dart';
@@ -333,14 +334,10 @@ String _borderStrokeToCssUnit(double value) {
   return '${value.toStringAsFixed(3)}px';
 }
 
-SVGSVGElement pathToSvgElement(
-    SurfacePath path, SurfacePaintData paint, String width, String height) {
+SVGSVGElement pathToSvgElement(SurfacePath path, SurfacePaintData paint) {
   // In Firefox some SVG typed attributes are returned as null without a
   // setter. So we use strings here.
-  final SVGSVGElement root = createSVGSVGElement()
-    ..setAttribute('width', '${width}px')
-    ..setAttribute('height', '${height}px')
-    ..setAttribute('viewBox', '0 0 $width $height');
+  final SVGSVGElement root = createSVGSVGElement();
 
   final SVGPathElement svgPath = createSVGPathElement();
   root.append(svgPath);
@@ -350,6 +347,9 @@ SVGSVGElement pathToSvgElement(
           paint.strokeWidth != null)) {
     svgPath.setAttribute('stroke', colorValueToCssString(paint.color)!);
     svgPath.setAttribute('stroke-width', '${paint.strokeWidth ?? 1.0}');
+    if (paint.strokeCap != null) {
+      svgPath.setAttribute('stroke-linecap', '${stringForStrokeCap(paint.strokeCap)}');
+    }
     svgPath.setAttribute('fill', 'none');
   } else if (paint.color != null) {
     svgPath.setAttribute('fill', colorValueToCssString(paint.color)!);

lib/web_ui/test/html/drawing/canvas_lines_golden_test.dart

@@ -17,13 +17,17 @@ Future<void> testMain() async {
   const Rect region = Rect.fromLTWH(0, 0, 300, 300);
 
   late BitmapCanvas canvas;
+  late BitmapCanvas domCanvas;
 
   setUp(() {
     canvas = BitmapCanvas(region, RenderStrategy());
+    // setting isInsideSvgFilterTree true forces use of DOM canvas
+    domCanvas = BitmapCanvas(region, RenderStrategy()..isInsideSvgFilterTree = true);
   });
 
   tearDown(() {
     canvas.rootElement.remove();
+    domCanvas.rootElement.remove();
   });
 
   test('draws lines with varying strokeWidth', () async {
@@ -33,6 +37,75 @@ Future<void> testMain() async {
     domDocument.body!.append(canvas.rootElement);
     await matchGoldenFile('canvas_lines_thickness.png', region: region);
   });
+  test('draws lines with varying strokeWidth with dom canvas', () async {
+
+    paintLines(domCanvas);
+
+    domDocument.body!.append(domCanvas.rootElement);
+    await matchGoldenFile('canvas_lines_thickness_dom_canvas.png', region: region);
+  });
+  test('draws lines with negative Offset values with dom canvas', () async {
+    // test rendering lines correctly with negative offset when using DOM
+    final SurfacePaintData paintWithStyle = SurfacePaintData()
+    ..color = 0xFFE91E63 // Colors.pink
+    ..style = PaintingStyle.stroke
+    ..strokeWidth = 16
+    ..strokeCap = StrokeCap.round;
+
+    // canvas.drawLine ignores paint.style (defaults to fill) according to api docs.
+    // expect lines are rendered the same regardless of the set paint.style
+    final SurfacePaintData paintWithoutStyle = SurfacePaintData()
+    ..color = 0xFF4CAF50 // Colors.green
+    ..strokeWidth = 16
+    ..strokeCap = StrokeCap.round;
+
+    // test vertical, horizontal, and diagonal lines
+    final List<Offset> points = <Offset>[
+      const Offset(-25, 50), const Offset(45, 50),
+      const Offset(100, -25), const Offset(100, 200),
+      const Offset(-150, -145), const Offset(100, 200),
+    ];
+    final List<Offset> shiftedPoints = points.map((Offset point) => point.translate(20, 20)).toList();
+
+    paintLinesFromPoints(domCanvas, paintWithStyle, points);
+    paintLinesFromPoints(domCanvas, paintWithoutStyle, shiftedPoints);
+
+    domDocument.body!.append(domCanvas.rootElement);
+    await matchGoldenFile('canvas_lines_with_negative_offset.png', region: region);
+  });
+
+  test('drawLines method respects strokeCap with dom canvas', () async {
+    final SurfacePaintData paintStrokeCapRound = SurfacePaintData()
+    ..color = 0xFFE91E63 // Colors.pink
+    ..strokeWidth = 16
+    ..strokeCap = StrokeCap.round;
+
+    final SurfacePaintData paintStrokeCapSquare = SurfacePaintData()
+    ..color = 0xFF4CAF50 // Colors.green
+    ..strokeWidth = 16
+    ..strokeCap = StrokeCap.square;
+
+    final SurfacePaintData paintStrokeCapButt = SurfacePaintData()
+    ..color = 0xFFFF9800 // Colors.orange
+    ..strokeWidth = 16
+    ..strokeCap = StrokeCap.butt;
+
+    // test vertical, horizontal, and diagonal lines
+    final List<Offset> points = <Offset>[
+      const Offset(5, 50), const Offset(45, 50),
+      const Offset(100, 5), const Offset(100, 200),
+      const Offset(5, 10), const Offset(100, 200),
+    ];
+    final List<Offset> shiftedPoints = points.map((Offset point) => point.translate(50, 50)).toList();
+    final List<Offset> twiceShiftedPoints = shiftedPoints.map((Offset point) => point.translate(50, 50)).toList();
+
+    paintLinesFromPoints(domCanvas, paintStrokeCapRound, points);
+    paintLinesFromPoints(domCanvas, paintStrokeCapSquare, shiftedPoints);
+    paintLinesFromPoints(domCanvas, paintStrokeCapButt, twiceShiftedPoints);
+
+    domDocument.body!.append(domCanvas.rootElement);
+    await matchGoldenFile('canvas_lines_with_strokeCap.png', region: region);
+  });
 }
 
 void paintLines(BitmapCanvas canvas) {
@@ -70,3 +143,10 @@ void paintLines(BitmapCanvas canvas) {
   paint3.color = 0xFFFF9800; // Colors.orange;
   canvas.drawLine(const Offset(50, 70), const Offset(150, 70), paint3);
 }
+
+void paintLinesFromPoints(BitmapCanvas canvas, SurfacePaintData paint, List<Offset> points) {
+  // points list contains pairs of Offset points, so for loop step is 2
+  for (int i = 0; i < points.length - 1; i += 2) {
+    canvas.drawLine(points[i], points[i + 1], paint);
+  }
+}