Well, I guess you can insert the following into your index.html template before the game's < script > element
<script>
(function() {
var base = HTMLCanvasElement.prototype.getContext;
HTMLCanvasElement.prototype.getContext = function(mode, attrs) {
if (attrs && typeof attrs == "object") attrs.preserveDrawingBuffer = true;
return base.apply(this, [].slice.call(arguments));
};
})();
</script>It's a pretty strange hack but whatever.
Went on and checked what's the deal anyway, subsequently filing a bug: