In latest master, syntax-propertize application seems to be buggy in a
way I can't readily reproduce or analyze.

Consider this bit of shell:

                elif [[ $c = '\' ]]; then
                    state=word-backslash

Sometimes, unreproducibly of course, we don't close the string at the
trailing
single quotation mark, and the rest of the buffer ends up being a
string, at least up until the single quotation mark. A similar problem
in Python sometimes leads to Python inflooping in buffers containing
strings triple-quoted with single quotation marks: it appears that we
interpret "'''" as just an empty single-quoted string, followed by a
single-quoted opener.

Did we change something about how complex syntax rules work lately?
Revision history doesn't point to anything.