Or you could use this.
Clicking the button will execute only on the current connection.
Clicking the button while holding down the Control key will execute on ALL connections.
Code:
on *:start:toolbar -az1n20 UnMark "Unhilight all" mirc.exe /unmarkall
alias unmarkall $iif($mouse.key & 2,scon -a) _unmarkall
alias -l _unmarkall var %i = 1 | while $window(*,%i) { window -g0 $qt($v1) | inc %i }