Fix retina scale when display size is implicit

The retinaScale helper now enforces the display size to the correct values because if no style has been set on the canvas, the render size is used as display size, making the chart bigger (or smaller) when deviceAspectRatio is different of 1.

src/core/core.controller.js

@@ -287,12 +287,11 @@ module.exports = function(Chart) {
 
 			canvas.width = chart.width = newWidth;
 			canvas.height = chart.height = newHeight;
-
-			helpers.retinaScale(chart);
-
 			canvas.style.width = newWidth + 'px';
 			canvas.style.height = newHeight + 'px';
 
+			helpers.retinaScale(chart);
+
 			// Notify any plugins about the resize
 			var newSize = {width: newWidth, height: newHeight};
 			Chart.plugins.notify('resize', [me, newSize]);

src/core/core.helpers.js

@@ -832,17 +832,24 @@ module.exports = function(Chart) {
 			document.defaultView.getComputedStyle(el, null).getPropertyValue(property);
 	};
 	helpers.retinaScale = function(chart) {
-		var ctx = chart.ctx;
-		var canvas = chart.canvas;
-		var width = canvas.width;
-		var height = canvas.height;
 		var pixelRatio = chart.currentDevicePixelRatio = window.devicePixelRatio || 1;
-
-		if (pixelRatio !== 1) {
-			canvas.height = height * pixelRatio;
-			canvas.width = width * pixelRatio;
-			ctx.scale(pixelRatio, pixelRatio);
+		if (pixelRatio === 1) {
+			return;
 		}
+
+		var canvas = chart.canvas;
+		var height = chart.height;
+		var width = chart.width;
+
+		canvas.height = height * pixelRatio;
+		canvas.width = width * pixelRatio;
+		chart.ctx.scale(pixelRatio, pixelRatio);
+
+		// If no style has been set on the canvas, the render size is used as display size,
+		// making the chart visually bigger, so let's enforce it to the "correct" values.
+		// See https://github.com/chartjs/Chart.js/issues/3575
+		canvas.style.height = height + 'px';
+		canvas.style.width = width + 'px';
 	};
 	// -- Canvas methods
 	helpers.clear = function(chart) {

test/core.controller.tests.js

@@ -93,7 +93,7 @@ describe('Chart.Controller', function() {
 			expect(data.datasets.length).toBe(0);
 		});
 
-		it('should NOT alter config.data references', function() {
+		it('should not alter config.data references', function() {
 			var ds0 = {data: [10, 11, 12, 13]};
 			var ds1 = {data: [20, 21, 22, 23]};
 			var datasets = [ds0, ds1];
@@ -218,7 +218,7 @@ describe('Chart.Controller', function() {
 			});
 		});
 
-		it('should NOT apply aspect ratio when height specified', function() {
+		it('should not apply aspect ratio when height specified', function() {
 			var chart = acquireChart({
 				options: {
 					responsive: false,
@@ -310,7 +310,7 @@ describe('Chart.Controller', function() {
 			});
 		});
 
-		it('should NOT inject the resizer element', function() {
+		it('should not inject the resizer element', function() {
 			var chart = acquireChart({
 				options: {
 					responsive: false
@@ -425,7 +425,7 @@ describe('Chart.Controller', function() {
 			});
 		});
 
-		it('should NOT include parent padding when resizing the canvas', function(done) {
+		it('should not include parent padding when resizing the canvas', function(done) {
 			var chart = acquireChart({
 				type: 'line',
 				options: {
@@ -625,7 +625,7 @@ describe('Chart.Controller', function() {
 			});
 		});
 
-		it('should NOT resize the canvas when parent height changes', function(done) {
+		it('should not resize the canvas when parent height changes', function(done) {
 			var chart = acquireChart({
 				options: {
 					responsive: true,
@@ -666,6 +666,53 @@ describe('Chart.Controller', function() {
 		});
 	});
 
+	describe('Retina scale (a.k.a. device pixel ratio)', function() {
+		beforeEach(function() {
+			this.devicePixelRatio = window.devicePixelRatio;
+			window.devicePixelRatio = 3;
+		});
+
+		afterEach(function() {
+			window.devicePixelRatio = this.devicePixelRatio;
+		});
+
+		// see https://github.com/chartjs/Chart.js/issues/3575
+		it ('should scale the render size but not the "implicit" display size', function() {
+			var chart = acquireChart({
+				options: {
+					responsive: false
+				}
+			}, {
+				canvas: {
+					width: 320,
+					height: 240,
+				}
+			});
+
+			expect(chart).toBeChartOfSize({
+				dw: 320, dh: 240,
+				rw: 960, rh: 720,
+			});
+		});
+
+		it ('should scale the render size but not the "explicit" display size', function() {
+			var chart = acquireChart({
+				options: {
+					responsive: false
+				}
+			}, {
+				canvas: {
+					style: 'width: 320px; height: 240px'
+				}
+			});
+
+			expect(chart).toBeChartOfSize({
+				dw: 320, dh: 240,
+				rw: 960, rh: 720,
+			});
+		});
+	});
+
 	describe('controller.destroy', function() {
 		it('should reset context to default values', function() {
 			var chart = acquireChart({});

test/mockContext.js

@@ -214,16 +214,19 @@
 				var chart = actual.chart;
 				var canvas = chart.ctx.canvas;
 				var style = getComputedStyle(canvas);
+				var pixelRatio = window.devicePixelRatio;
 				var dh = parseInt(style.height, 10);
 				var dw = parseInt(style.width, 10);
 				var rh = canvas.height;
 				var rw = canvas.width;
+				var orh = rh / pixelRatio;
+				var orw = rw / pixelRatio;
 
 				// sanity checks
-				if (chart.height !== rh) {
-					message = 'Expected chart height ' + chart.height + ' to be equal to render height ' + rh;
-				} else if (chart.width !== rw) {
-					message = 'Expected chart width ' + chart.width + ' to be equal to render width ' + rw;
+				if (chart.height !== orh) {
+					message = 'Expected chart height ' + chart.height + ' to be equal to original render height ' + orh;
+				} else if (chart.width !== orw) {
+					message = 'Expected chart width ' + chart.width + ' to be equal to original render width ' + orw;
 				}
 
 				// validity checks