Here is the first version of the mode with the behavior like in zsh,
but that is based on icomplete-mode.  So many customizable variables
and commands were copied from icomplete-mode, but instead of displaying
completions as an overlay in the minibuffer, in zcomplete-mode
completions are displayed in the standard *Completions* window.
All other icomplete features are preserved in zcomplete, such as
typing in the minibuffer continuously updates a list of possible
completions that match the typed string: