今天碰到一个问题，就是在多屏的时候，WM_NCHITTEST结果不正确，经检查发现在多屏的情况下，鼠标的位置是有可能出现负值的，但是如果还是用LoWord(Msg.LParam)和HiWORD(Msg.LParam)取鼠标的位置，就会取出不正确的值，因为这两个函数只会返回正值。
查询MSDN的WM_NCHITTEST有下面一段话。
Remarks
Use the following code to obtain the horizontal and vertical position:
xPos = GET_X_LPARAM(lParam); yPos = GET_Y_LPARAM(lParam);
As noted above, the x-coordinate is in the low-order short of the return value; the y-coordinate is in the high-order short (both represent signed values because they can take negative values on systems with multiple monitors). If the return value is assigned to a variable, you can use the MAKEPOINTS macro to obtain a POINTS structure from the return value. You can also use the GET_X_LPARAM or GET_Y_LPARAM macro to extract the x- or y-coordinate.
Important Do not use the LOWORD or HIWORD macros to extract the x- and y- coordinates of the cursor position because these macros return incorrect results on systems with multiple monitors. Systems with multiple monitors can have negative x- and y- coordinates, and LOWORD and HIWORD treat the coordinates as unsigned quantities.
在delphi中可通过下面代码获取正确的鼠标位置：
LMousePt.X := SmallInt(Msg.LParamLo); LMousePt.Y := SmallInt(Msg.LParamHi); //或者 LMousePt.X := SmallInt(Msg.LParam); LMousePt.Y := (SmallInt)HiWord(Msg.LParam);
