a function to convert client-area-relative cooordinates to window-relative coordinates

ClientToWindow.ahk

ClientToWindow(hwnd, byRef x, byRef y)
{
	VarSetCapacity(pt, 8, 0)
	, NumPut(x, pt, 0, "UInt")
	, NumPut(y, pt, 4, "UInt")

	if (!DllCall("ClientToScreen", "Ptr", hwnd, "Ptr", &pt, "UInt"))
		return false

	VarSetCapacity(rc, 16, 0)
	if (!DllCall("GetWindowRect", "Ptr", hwnd, "Ptr", &rc, "UInt"))
		return false

	screen_x := NumGet(pt, 0, "UInt")
	, screen_y := NumGet(pt, 4, "UInt")

	win_x := NumGet(rc, 0, "UInt")
	, win_y := NumGet(rc, 4, "UInt")

	x := screen_x - win_x
	, y := screen_y - win_y

	return true
}