My version of resize_split.pl is a slight modification of David Leadbeater's.
From the email I sent him at the end of 2003:
I had a problem with resize_split for Irssi: when a status bar is only present when a window is active, the change of size of the windows was wrong and the windows were eventually arriving being the same size. I have added a resize_num_window_actonly setting to indicate the number of those. By default, the behavior isn't changed. In my case, setting the variable to 1 makes resize_split behave the right way. Ideally, this is something that should be gotten from Irssi directly rather than from the user, but I haven't figured out how to do so.
This could be cleaner, but I couldn't figure out how to get the information needed from the Irssi API.