设置窗口顶层：
SetWindowLong(hWnd,GWL_EXSTYLE,GetWindowLong(hWnd,GWL_EXSTYLE) | WS_EX_TOPMOST);
SetWindowPos(hWnd,HWND_TOPMOST,0,0,0,0,SWP_NOMOVE | SWP_NOSIZE);
If you have changed certain window data using SetWindowLong, you must
call SetWindowPos to have the changes take effect.
取消窗口顶层：
SetWindowPos(hWnd,HWND_NOTOPMOST ,0,0,0,0,SWP_NOMOVE | SWP_NOSIZE);
The SetWindowPos function changes the size, position, and Z order of a
child, pop-up, or top-level window. Child, pop-up, and top-level windows are
ordered according to their appearance on the screen. The topmost window receives
the highest rank and is the first window in the Z order.
BOOL SetWindowPos(
HWND hWnd, // handle to window
HWND hWndInsertAfter, // placement-order handle
int X, // horizontal position
int Y, // vertical position
int cx, // width
int cy, // height
UINT uFlags // window-positioning flags
);