Unexpected stdin-behavior

[Tycho Kirchner <tychokirchner@xxxxxxx>]

Dear zsh-maintainers,

first of all, I'm not a mail-subscriber but please respond to this email 
anyway ^_^

I almost finished the (yet unpublished) zsh-integration of my 
shell-tracker shournal[1] and noticed some inconsistency on the handling 
of stdin during an integration test.
While bash -i and zsh -s show the expected behavior (stdin is "consumed" 
command by command) zsh -s -i seems to "consume" the whole stdin 
beforehand (see below). I there a rationale behind this?
What I actually try to do is running automated tests on an interactive 
zsh with prompts, preexec, etc by piping commands to it similar to the 
snippet below.

Thanks in advance
Tycho Kirchner

[1] https://github.com/tycho-kirchner/shournal

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debian-dell% echo $ZSH_VERSION
5.8
debian-dell% stdintest(){ printf 'echo one\nsh -c "while read -r row; do 
echo got row \$row; done" \necho two\n'; }
debian-dell% stdintest | bash  --norc --noprofile -i
bash-5.0$ echo one
one
bash-5.0$ sh -c "while read -r row; do echo got row \$row; done"
got row echo two
bash-5.0$ exit
debian-dell% stdintest | zsh -f -s
one
got row echo two
debian-dell% stdintest | zsh -f -s -i
debian-dell% one
debian-dell% %
debian-dell% two
debian-dell% %
debian-dell%