Juri Linkov <juri@linkov.net> writes:
>> This is because elisp-completion-at-point returns a completion region
>> covering the entire sexp after the start of the region.  That sexp is
>> "(progn (some) (big) (expression))" in the first case and "a" in the
>> second case.
>
> This is a known bug without a known fix:
> https://debbugs.gnu.org/64903#18

Alright, how about this patch?  Should fix most of the issue without
changing the code too much.