CoordMode, ToolTip, Screen
DllCall("RegisterShellHookWindow", "UInt", A_ScriptHwnd) 
OnMessage(DllCall("RegisterWindowMessage", "str", "SHELLHOOK"), "ShellProc") 
OnExit, Exit
Return

ShellProc(nCode, wParam)
{ 
    static A := 1
    If (nCode = 4)
    { 
        WinGet, Process, ProcessName, % !wParam ? "A" : "ahk_id " wParam 
        ToolTip % A++ "`n`n" Process, 15, 15
    }    
} 

Exit:
    DllCall("DeregisterShellHookWindow", "UInt", A_ScriptHwnd)
    ExitApp

#Persistent
EVENT_OBJECT_FOCUS := 0x8005
global WS_CHILD := 0x40000000, hPrevActive

WinGet, hPrevActive,, A

HWINEVENTHOOK := SetWinEventHook(EVENT_OBJECT_FOCUS, EVENT_OBJECT_FOCUS, 0
                     , RegisterCallback("WatchingActivateWindow", "F"), 0, 0, 0)
                     
OnExit, Exit
return

Exit:
   DllCall("UnhookWinEvent", Ptr, HWINEVENTHOOK)
   ExitApp
   
WatchingActivateWindow(hWinEventHook, event, hwnd)
{
   h := hwnd
   While !toplevel
   {
      WinGet, Style, Style, ahk_id %h%
      if (Style & WS_CHILD)
         h := DllCall("GetParent", Ptr, h, Ptr)
      else
         toplevel := 1
   }
   
   if (h != hPrevActive)
      ToolTip % hPrevActive := h   ; хэндл активного в данный момент окна
}

SetWinEventHook(eventMin, eventMax, hmodWinEventProc, lpfnWinEventProc, idProcess, idThread, dwFlags)
{
   return DllCall("SetWinEventHook" , UInt, eventMin, UInt, eventMax
                                    , Ptr, hmodWinEventProc, Ptr, lpfnWinEventProc
                                    , UInt, idProcess, UInt, idThread
                                    , UInt, dwFlags, Ptr)
}