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.
当用户按住鼠标移动时，光标会经过源对象程序内部或者外部的一系列对象。
As the user moves the mouse around with the button held down, the cursor passes over a variety of objects.
用户移动鼠标，直到屏幕上的光标指向所期望的对象或置于对象之上。
A user moves the mouse until the onscreen cursor is pointing to, or placed over, the desired object.
改变它的外观，当鼠标光标移到对象上时，对象会显示这种特性。
Objects that can be directly manipulated often change their appearance subtly to indicate this attribute when the mouse cursor moves over them.
改变它的外观，当鼠标光标移到对象上时，对象会显示这种特性。
Objects that can be directly manipulated often change their appearance subtly to indicate this attribute when the mouse cursor moves over them.
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