If you hold either mouse key (Tested with right and left keys, I don't have a middle one) and then leave a window, mIRC will continuously send leave events until you let go of the key, to reproduce:

Code:
alias testing { window -dp @test }
menu @test {
  leave:{
    echo -s Left $leftwin
  }
}


Type /testing, then press and hold either key inside @test, move out of the window whilst still holding the key and then look in the status window, it should of flooded with "Left @test".