VC ScreenToClient与ClientToScreen

The ClientToScreen function replaces theclient-area coordinates in the POINT structure with the screen coordinates. The screen coordinates are relative to the upper-left corner of the screen. Note, a screen-coordinate point that is above the window's client area has a negative y-coordinate. Similarly, a screen coordinate to the left of a client area has a negative x-coordinate.

The ScreenToClient member function replaces the screen coordinates given in lpPoint or lpRect with client coordinates. The new coordinates are relative to the upper-left corner of the CWnd client area.

实际就是一个屏幕坐标，一个客户区坐标，这两个坐标系统的转换

GetWindowRect得到的是screen coordinates.  GetClientRect得到的是client-area coordinates，所以GetClientRect得到的总是以（0，0）为左上的点。

The dimensions are given in screen coordinates relative to the upper-left corner of thedisplay screen. The dimensions of the caption, border, and scroll bars, if present, are included.

如果我们要获取一个窗口pWnd相对于父窗口parent客户区的位置应该是：

Rect rc;

pWnd->GetWindowRect(&rc);

parent->ScreenToClient(&rc);

如果如下：

pWnd->GetWindowRect(&rc);

pWnd->ScreenToClient(&rc);

得到的rc左顶坐标会出项负数，因为将自己的窗口往客户区映射，会出现标题栏/边框等的位置差错。