The cursor is drawn in each of the windows differently depending on the item being selected. You can isolate a function that resembles the original but adjust the rectangle value.
Check this part:
look at how rect.width is being drawn. It is drawn based on the width of the item it is being selected at.
Check this part:
Window_Selectable.prototype.itemRect = function(index) {
var rect = new Rectangle();
var maxCols = this.maxCols();
rect.width = this.itemWidth();
rect.height = this.itemHeight();
rect.x = index % maxCols * (rect.width + this.spacing()) - this._scrollX;
rect.y = Math.floor(index / maxCols) * rect.height - this._scrollY;
return rect;
};
look at how rect.width is being drawn. It is drawn based on the width of the item it is being selected at.