最近hook滚动条，用到了很多这方面小知道，总结下:
In the border of a window that does not have a sizing border.
边界上！that does not have a sizing border好像没什么关系，反正普通的框鼠标停在边上就有这个事件(自测试）
In the lower-horizontal border of a resizable window (the user can click the mouse to resize the window vertically).
可调窗体大小的下边界
In the lower-left corner of a border of a resizable window (the user can click the mouse to resize the window diagonally).
可调窗体大小的左下角
In the lower-right corner of a border of a resizable window (the user can click the mouse to resize the window diagonally).
可调窗体大小的右下角
In the left border of a resizable window (the user can click the mouse to resize the window horizontally).
可调窗体大小的左边界
In the right border of a resizable window (the user can click the mouse to resize the window horizontally).
可调窗体大小的右边界
In the upper-horizontal border of a window.
窗体的上边界
In the upper-left corner of a window border.
窗体的左上角
窗体的右上角
In a title bar.
标题栏,用来模拟拖动区
In a client area.
客户区
In a Close button
关闭按钮上
On the screen background or on a dividing line between windows (same as HTNOWHERE, except that the DefWindowProc function produces a system beep to indicate an error).
在屏幕的后面或在窗体之间的线上（相当于HTNOWHERE，除了DefWindowProc函数产生的”嘟嘟”提示)
In a size box (same as HTSIZE).
不懂了？？？？？？？？？？？？？？？？？？？？？？？
In a Help button.
帮助按钮上
In a horizontal scroll bar.
水平滚动条上
In a menu.
菜单上
In a Maximize button.
最大化按钮上
In a Minimize button.
最小化按钮上
On the screen background or on a dividing line between windows
在屏幕的后面或在窗体之间的线上，测试应该就是在窗口边界处有这个事件
In a Minimize button.
最小化的按钮，就是HTMINBUTTON
In a size box (same as HTGROWBOX).
不懂了？？？？？？？？？？？？？？？？？？？？？？？
In a window menu or in a Close button in a child window.
在一个系统菜单上,或在一个子窗体的一个关闭按钮上
In a window currently covered by another window in the same thread (the message will be sent to underlying windows in the same thread until one of them returns a code that is not HTTRANSPARENT).
在同一线程的两个窗体，一个窗体被另一个窗体覆盖着（消息会传递到下面的窗体，直到其中的一个窗体离开，否则就发送 HTTRANSPARENT消息）
In the vertical scroll bar.
在垂直滚动条上