// in Enjine.KeyboardInput 
  Initialize: function() {
    var self = this;
    document.onkeydown = function(event) { self.KeyDownEvent(event); }
    document.onkeyup = function(event) { self.KeyUpEvent(event); }
  },
  IsKeyDown: function(key) {
    if (this.Pressed[key] != null)
      return this.Pressed[key];
    return false;
  },
    
  KeyDownEvent: function(event) {
    this.Pressed[event.keyCode] = true;
  },
  
  KeyUpEvent: function(event) {
    this.Pressed[event.keyCode] = false;
  }

Sheesh. That's why it's jumping up and down when you use the arrow keys. All keyboard input is based on checking `IsKeyDown`, and nothing else. `event.preventDefault()` please. Why do people do this? .preventDefault() is missing in a huge amount of HTML5 demos that aren't full-screen; surely it's not that obscure.

edit: seriously, that's all there is to it. Pop open the console / inspector in your browser of choice, and run this:

  document.onkeyup = function(event) { Enjine.KeyboardInput.KeyUpEvent(event); event.preventDefault(); }
  document.onkeydown = function(event) { Enjine.KeyboardInput.KeyDownEvent(event); event.preventDefault(); }

Viola. No more jumping. No more keyboard scrolling either, which is why you check if the cursor location / last-clicked location is within your canvas.

edit2: that aside, very nice work. Smooth playing and so far no glitches, and highly readable code :)

edit3: if you're on IE, that code is `event.returnValue = false` btw. Yaaay, IE :|