Hello,
I'm debugging the reason why Ctrl-C can be ignored. U stumbled upon this:

1. I open shell, enter "ls ", then press Ctrl-C
2. Debug logs from before "< master Zle select()" and after "> master
Zle select()" occur
3. At first Ctrl-C, there are 24 select() recurrent code execution
paths – the logs around select() are triggered ~24 times.
4. At second "ls <Ctrl-C>", there are 96 recurrent code paths.
5. At third "ls <Ctrl-C>, there are 197 recurrent code paths.

The logs are:

BASIC < select Before: selret: 0, errflag: 0, retflag: 0, breaks: 0,
exit_pending: 0
BASIC > select After: selret: 0, errflag: 0, retflag: 0, breaks: 0,
exit_pending: 0

two lines, installed before master select() (first line) and after the
master select (second line).

Could it be that Zsh reacts in this way after occurring Ctrl-C? It's
somewhat hard to believe. That's why I include also patch with debug
prints that I've used. Maybe someone will just apply it and do "ls
<Ctrl-C>".

Also: asciinema recording, showing how the series of logs grow (1 tmux
screen / page, then 4 tmux screens / pages, then 9 pages to scroll up
to see the logs beginnings'):

https://asciinema.org/a/hC24KqphbztwQfk5ApZbOYU2y

-- 
Best regards,
Sebastian Gniazdowski