Another vote for ESC cancelling the current construction.

It's just a new condition in onkeydown:

  else if (e.keyCode === 27) {
    deselect();
  }

While we're at it, make Shift+z redo.

  else if (e.keyCode === 90) {
      //"z"
      if (e.shiftKey) {
          changes.redo();
      }
      else {
          changes.undo();
          changes.record(1);
      }
  }

And let's not clutter up the user's history. Swapping

  window.location.hash=currenthash;
  ...
  window.location.hash=hash;

with

  window.location.replace(currenthash);
  ...
  window.location.replace(hash);

fixes that one.

It's the little things in life, man…