补充：
Unfortunately, there is no function to retrieve the thumb's 32-bit position
while the user moves it. GetScrollPos provides static position data only;
an application can therefore only obtain 32-bit position data before or
after a scroll has taken place.
不幸的是，移动过程中没有办法得到数据，GetScrollPos只能在移动之前或者
之后得到静态的数据
