Certainly would be handy. I could swear there was already a way to do that but I can't see it.
Not that this affects the validity of your suggestion, but if you just need to know if a window has a side-listbox or not you can use something like:
alias has_side_listbox return $iif($window($1).type == custom && $line($1, 0, 1) !== $null, $true, $false)
Of course that's far from an obvious and convenient way to get that information and it still doesn't provide the actual width of the listbox so a $window() property would be very useful.
