该函数做很多位置整理(position - munging)工作，但都可归结为一个简单的思想：不断保存鼠标光标与目标对象之间的精确的距离。
This function does a good bit of position-munging, but it all boils down to a simple idea: Keep constant the precise distance between the mouse cursor and the target object.
但是，对拖动的处理必须在文档对象中运行，因为要获得所有鼠标移动事件，而不仅仅是光标在目标对象上时发生的鼠标事件。
The drag handling, however, must be running in the document object, because you want to get all mouse-move events, not just those that might happen with the cursor over the target object.
指定在运行时刻，当鼠标位于对象的某一特定部分时，用作自定义鼠标指针的光标文件。
Specifies the cursor file as a customized mouse pointer when the mouse is over a particular part of an object at run time.
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