code: function charWidth(display) {
      if (display.cachedCharWidth != null) return display.cachedCharWidth;
      var anchor = elt("span", "xxxxxxxxxx");
      var pre = elt("pre", [anchor]);
      removeChildrenAndAdd(display.measure, pre);
      var rect = anchor.getBoundingClientRect(), width = (rect.right - rect.left) / 10;
      if (width > 2) display.cachedCharWidth = width;
      return width || 10;
    }
docs: Compute the default character width.