So inserting a letter from the Completions buffer is the only thing
that you miss from zsh?
Actually yes, that and remove the mode-line to completions...
But you also said above that this feature is
not important because such cases are rare: "M-x find-file /my/file/path".
I can live with that. Lets say in our case adding a space does not make
much sense, but just inserting the letter in place without completion
may be a better behavior... But again... I can live with this.
Indeed, there is a problem with staying in the Completions buffer
and typing a letter. When this letter will update the completion,
then the Completions buffer should be updated that causes more problems
such as where to move point after such buffer reverting, since
the Completions buffer is the current buffer, etc.
In all other regards, currently it works exactly like in zsh:
'TAB TAB' selects the Completions window, and C-g cancels it.
Yes...
So we have two possible cases: selecting a completion candidate
when the Completions buffer is the current buffer, and the second
case is when the minibuffer is current. The first case now works fine,
and we need to improve the second case. The new mode zcomplete-mode
will help to do this.