This is crucial, because the user might drag the cursor all over the page during the drag-and-drop sequence.
这一点十分关键，因为在拖放过程中用户可能会将光标拖过整个页面。
The drag handlers are uninstalled, leaving the original onmousedown handlers behind, ready to start the next drag-and-drop sequence.
这个拖动处理程序将被卸载，只留下最初的 onmousedown 处理程序，准备好开始下一轮的拖放过程。
However, at first, you only want to install the mousedown handler, because you're still waiting for the click that initiates the drag-and-drop sequence.
不过，开始时，只需安装mousedown处理程序，因为此时您尚在等待激发初始化拖放过程的单击。
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