Code:
menu @Window { 
  dclick { 
    if ($line(@Window,1,1) != $null) { 
      do some stuff
    }
  }
}


Unless you always have "0" in line 1 if it has lines, use $null. Also, no need to halt something like this. If it's $null, it won't have another other commands to do anyhow. If you DO want to halt something, use RETURN unless you're halting mIRC's commands/outputs rather than the script.

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