SetTimer, Activation, 500

~LButton:: SetTimer, Activation, Off
LButton Up:: SetTimer, Activation, On

Activation:
  MouseGetPos, X, Y, Win
  If(Win != PrevWin)
  {
    PrevWin := Win
    WinActivate, ahk_id %Win%
  }
Return

Loop
{
  MouseGetPos, X, Y, Win
  If(Win != PrevWin)
  {
    PrevWin := Win
    WinActivate, ahk_id %Win%
  }
  Sleep, 500
}

F12:: Pause        ; Приостановка / возобновление по F12.