Then it's time to ask for listening for various mouse events (mousedown, mouseup, and click) on the shadow tree's rectangle — you'll see how to react to these later.
接下来要求对影像树的矩形监听各种不同的鼠标事件(mousedown、mouseup和click)，后面将看到如何响应这些事件。
When that click comes, you then want to install the mousemove and mouseup handlers.
当单击发生时，就需要安装 mousemove 和 mouseup 处理程序。
If they release it over another page element with the mouseup (fn) defined, what behavior should take place?
如果用户用所定义的mouseup (fn)在另一个页面元素之上释放鼠标，那么应该发生什么行为呢？
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