Selection：extend() 方法
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
Selection.extend() 方法会将选区的焦点移动到指定的位置。选区的锚点不会发生改变。无论方向如何，最终形成的选区范围都是从锚点到新的焦点。