fix buffer usage in export for stage and layer. fix #1903

src/Container.ts

@@ -120,8 +120,6 @@ export abstract class Container<
    * layer.add(shape1, shape2, shape3);
    * // empty arrays are accepted, though each individual child must be defined
    * layer.add(...shapes);
-   * // remember to redraw layer if you changed something
-   * layer.draw();
    */
   add(...children: ChildType[]) {
     if (children.length === 0) {

src/Layer.ts

@@ -385,7 +385,7 @@ export class Layer extends Container<Group | Shape> {
     // empty pixel
     return {};
   }
-  drawScene(can?: SceneCanvas, top?: Node) {
+  drawScene(can?: SceneCanvas, top?: Node, bufferCanvas?: SceneCanvas) {
     const layer = this.getLayer(),
       canvas = can || (layer && layer.getCanvas());
 
@@ -397,7 +397,7 @@ export class Layer extends Container<Group | Shape> {
       canvas.getContext().clear();
     }
 
-    Container.prototype.drawScene.call(this, canvas, top);
+    Container.prototype.drawScene.call(this, canvas, top, bufferCanvas);
 
     this._fire(DRAW, {
       node: this,

src/shapes/Image.ts

@@ -163,7 +163,6 @@ export class Image extends Shape<ImageConfig> {
    *  Konva.Image.fromURL(imageURL, function(image){
    *    // image is Konva.Image instance
    *    layer.add(image);
-   *    layer.draw();
    *  });
    */
   static fromURL(

test/unit/Shape-test.ts

@@ -1550,6 +1550,53 @@ describe('Shape', function () {
     compareCanvases(canvas2, canvas1, 240, 110);
   });
 
+  it('export stage when buffer canvas is for line', async function () {
+    var stage = addStage();
+
+    var layer = new Konva.Layer();
+    stage.add(layer);
+
+    const group = new Konva.Group({
+      id: 'group01',
+      draggable: false,
+      opacity: 0.99,
+    });
+    layer.add(group);
+
+    const arrow = new Konva.Arrow({
+      x: 0,
+      y: 0,
+      points: [50, 25, 200, 25, 200, 225, 400, 225],
+      stroke: 'purple',
+      fill: 'purple',
+      strokeWidth: 4,
+      pointerAtEnding: true,
+      bezier: true,
+    });
+    group.add(arrow);
+
+    const bounds = layer.getClientRect({ relativeTo: stage });
+    const pos = stage.getPosition();
+
+    const canvas1 = layer.toCanvas({
+      pixelRatio: 1,
+      x: bounds.x + pos.x,
+      y: bounds.y + pos.y,
+      width: bounds.width,
+      height: bounds.height,
+    });
+    group.opacity(1);
+    const canvas2 = layer.toCanvas({
+      pixelRatio: 1,
+      x: bounds.x + pos.x,
+      y: bounds.y + pos.y,
+      width: bounds.width,
+      height: bounds.height,
+    });
+
+    compareCanvases(canvas1, canvas2, 240, 110);
+  });
+
   // ======================================================
   it('optional disable shadow for stroke', function () {
     var stage = addStage();