fix: using existing canvas option

examples/existing_canvas.html

@@ -42,7 +42,7 @@ <h1>Existing canvas:</h1>
     ctx.stroke();
 
     document.querySelector("button").addEventListener("click", function() {
-        html2canvas(document.querySelector("#content"), {canvas: canvas}).then(function(canvas) {
+        html2canvas(document.querySelector("#content"), {canvas: canvas, scale: 1}).then(function(canvas) {
             console.log('Drew on the existing canvas');
         });
     }, false);

src/__tests__/index.ts

@@ -0,0 +1,91 @@
+import html2canvas from '../index';
+
+import {CanvasRenderer} from '../render/canvas/canvas-renderer';
+import {DocumentCloner} from '../dom/document-cloner';
+import {COLORS} from '../css/types/color';
+
+jest.mock('../core/logger');
+jest.mock('../css/layout/bounds');
+jest.mock('../dom/document-cloner');
+jest.mock('../dom/node-parser', () => {
+    return {
+        isBodyElement: () => false,
+        isHTMLElement: () => false,
+        parseTree: jest.fn().mockImplementation(() => {
+            return {styles: {}};
+        })
+    };
+});
+
+jest.mock('../render/stacking-context');
+jest.mock('../render/canvas/canvas-renderer');
+
+describe('html2canvas', () => {
+    // eslint-disable-next-line @typescript-eslint/no-object-literal-type-assertion
+    const element = {
+        ownerDocument: {
+            defaultView: {
+                pageXOffset: 12,
+                pageYOffset: 34
+            }
+        }
+    } as HTMLElement;
+
+    it('should render with an element', async () => {
+        DocumentCloner.destroy = jest.fn().mockReturnValue(true);
+        await html2canvas(element);
+        expect(CanvasRenderer).toHaveBeenLastCalledWith(
+            expect.objectContaining({
+                backgroundColor: 0xffffffff,
+                scale: 1,
+                height: 50,
+                width: 200,
+                x: 0,
+                y: 0,
+                scrollX: 12,
+                scrollY: 34,
+                canvas: undefined
+            })
+        );
+        expect(DocumentCloner.destroy).toBeCalled();
+    });
+
+    it('should have transparent background with backgroundColor: null', async () => {
+        await html2canvas(element, {backgroundColor: null});
+        expect(CanvasRenderer).toHaveBeenLastCalledWith(
+            expect.objectContaining({
+                backgroundColor: COLORS.TRANSPARENT
+            })
+        );
+    });
+
+    it('should use existing canvas when given as option', async () => {
+        // eslint-disable-next-line @typescript-eslint/no-object-literal-type-assertion
+        const canvas = {} as HTMLCanvasElement;
+        await html2canvas(element, {canvas});
+        expect(CanvasRenderer).toHaveBeenLastCalledWith(
+            expect.objectContaining({
+                canvas
+            })
+        );
+    });
+
+    it('should not remove cloned window when removeContainer: false', async () => {
+        DocumentCloner.destroy = jest.fn();
+        await html2canvas(element, {removeContainer: false});
+        expect(CanvasRenderer).toHaveBeenLastCalledWith(
+            expect.objectContaining({
+                backgroundColor: 0xffffffff,
+                scale: 1,
+                height: 50,
+                width: 200,
+                x: 0,
+                y: 0,
+                scrollX: 12,
+                scrollY: 34,
+                canvas: undefined
+            })
+        );
+        expect(DocumentCloner.destroy).not.toBeCalled();
+    });
+});

src/core/__mocks__/logger.ts

@@ -0,0 +1,17 @@
+export class Logger {
+    debug() {}
+
+    static create() {}
+
+    static destroy() {}
+
+    static getInstance(): Logger {
+        return logger;
+    }
+
+    info() {}
+
+    error() {}
+}
+
+const logger = new Logger();

src/css/layout/__mocks__/bounds.ts

@@ -0,0 +1,4 @@
+export const {Bounds} = jest.requireActual('../bounds');
+export const parseBounds = () => {
+    return new Bounds(0, 0, 200, 50);
+};

src/dom/__mocks__/document-cloner.ts

@@ -0,0 +1,16 @@
+export class DocumentCloner {
+    clonedReferenceElement?: HTMLElement;
+
+    constructor() {
+        // eslint-disable-next-line @typescript-eslint/no-object-literal-type-assertion
+        this.clonedReferenceElement = {} as HTMLElement;
+    }
+
+    toIFrame() {
+        return Promise.resolve({});
+    }
+
+    static destroy() {
+        return true;
+    }
+}

src/dom/document-cloner.ts

@@ -410,6 +410,14 @@ export class DocumentCloner {
                 : ` ${PSEUDO_HIDE_ELEMENT_CLASS_AFTER}`;
         return anonymousReplacedElement;
     }
+
+    static destroy(container: HTMLIFrameElement): boolean {
+        if (container.parentNode) {
+            container.parentNode.removeChild(container);
+            return true;
+        }
+        return false;
+    }
 }
 
 enum PseudoElementType {

src/index.ts

@@ -116,6 +116,7 @@ const renderElement = async (element: HTMLElement, opts: Partial<Options>): Prom
     const renderOptions = {
         id: instanceName,
         cache: options.cache,
+        canvas: options.canvas,
         backgroundColor,
         scale: options.scale,
         x: options.x,
@@ -153,7 +154,7 @@ const renderElement = async (element: HTMLElement, opts: Partial<Options>): Prom
     }
 
     if (options.removeContainer === true) {
-        if (!cleanContainer(container)) {
+        if (!DocumentCloner.destroy(container)) {
             Logger.getInstance(instanceName).error(`Cannot detach cloned iframe as it is not in the DOM anymore`);
         }
     }
@@ -163,11 +164,3 @@ const renderElement = async (element: HTMLElement, opts: Partial<Options>): Prom
     CacheStorage.destroy(instanceName);
     return canvas;
 };
-
-const cleanContainer = (container: HTMLIFrameElement): boolean => {
-    if (container.parentNode) {
-        container.parentNode.removeChild(container);
-        return true;
-    }
-    return false;
-};

src/render/canvas/canvas-renderer.ts

@@ -71,10 +71,12 @@ export class CanvasRenderer {
         this.canvas = options.canvas ? options.canvas : document.createElement('canvas');
         this.ctx = this.canvas.getContext('2d') as CanvasRenderingContext2D;
         this.options = options;
-        this.canvas.width = Math.floor(options.width * options.scale);
-        this.canvas.height = Math.floor(options.height * options.scale);
-        this.canvas.style.width = `${options.width}px`;
-        this.canvas.style.height = `${options.height}px`;
+        if (!options.canvas) {
+            this.canvas.width = Math.floor(options.width * options.scale);
+            this.canvas.height = Math.floor(options.height * options.scale);
+            this.canvas.style.width = `${options.width}px`;
+            this.canvas.style.height = `${options.height}px`;
+        }
         this.fontMetrics = new FontMetrics(document);
         this.ctx.scale(this.options.scale, this.options.scale);
         this.ctx.translate(-options.x + options.scrollX, -options.y + options.scrollY);

www/src/preview.ts

@@ -42,8 +42,10 @@ function onBrowserChange(browserTest: Test) {
         previewImage.src = `/results/${browserTest.screenshot}.png`;
         if (browserTest.devicePixelRatio > 1) {
             previewImage.style.transform = `scale(${1 / browserTest.devicePixelRatio})`;
+            previewImage.style.transformOrigin = 'top left';
         } else {
             previewImage.style.transform = '';
+            previewImage.style.transformOrigin = '';
         }
     }
 }