[HTML5] Fix DPI-handling

Backends/HTML5/kha/SystemImpl.hx

@@ -533,9 +533,22 @@ class SystemImpl {
 
 			if (canvas.getContext != null) {
 				if (lastCanvasClientWidth != canvas.clientWidth || lastCanvasClientHeight != canvas.clientHeight) {
+					// canvas.width is the actual backbuffer-size.
+					// canvas.clientWidth (which is read-only and equivalent to
+					// canvas.style.width in pixels) is the output-size
+					// and by default gets scaled by devicePixelRatio.
+					// We revert that scale so backbuffer and output-size
+					// are the same.
+
 					var scale = Browser.window.devicePixelRatio;
-					canvas.width = Std.int(canvas.clientWidth * scale);
-					canvas.height = Std.int(canvas.clientHeight * scale);
+					var clientWidth = canvas.clientWidth;
+					var clientHeight = canvas.clientHeight;
+					canvas.width = clientWidth;
+					canvas.height = clientHeight;
+					if (scale != 1) {
+						canvas.style.width = Std.int(clientWidth / scale) + "px";
+						canvas.style.height = Std.int(clientHeight / scale) + "px";
+					}
 					lastCanvasClientWidth = canvas.clientWidth;
 					lastCanvasClientHeight = canvas.clientHeight;
 				}