Question mark before or after parentheses for incidental text? [duplicate]

Regarding "What happens when there's a question mark and only part of the sentence is a question?" and your three suggestions, my opinions are:

• I was distracted by a plane (or was it Superman?) ‒ Is missing period.
• I was distracted by a plane (or was it Superman?). ‒ Is ok
• I was distracted by a plane (or was it Superman)? ‒ Treats whole as question

I don't have definitive authorities to support the above choices. I have seen only three webpages that even remotely address your question:

• englishplus says

  In a sentence containing a parenthetical expression, any punctuation belonging to the main sentence goes outside the parentheses.

• wowwritingworkshop says

  A sentence in parentheses within another sentence does not take a period, because the period is reserved for the main sentence (questions and exclamations, however, must have their respective marks!).

• brians at public.wsu.edu says

  Normally a sentence’s final punctuation mark—whether period, exclamation point, or question mark—goes outside such a parenthesis (like this). However, if the material inside the parenthesis requires a concluding punctuation mark like an exclamation point or question mark (but not a period!), that mark is placed inside the closing mark even though another mark is outside it. This latter sort of thing is awkward, however, and best avoided if you can help it.

Note, rather than using parentheses I would word the sentence as

I was distracted by a plane; or was it Superman?

When there's a question mark in the parenthetical matter and only part of the sentence is a question, you should put the question mark inside the parentheses and a period (full stop) at the end of the sentence:

I was distracted by a plane (or was it Superman?).

You asked for a reason why. The reason comes from the purpose for parentheses. From The Grammar Bible by Michael Strumpf: Parentheses enclose words, phrases, clauses, and sentences that are included in a sentence as explanations or commentary .... Punctuate the primary portion of the sentence as if the parenthetical portion were not there.

You should be able to lift out the parenthetical portion and still have the sentence make sense and be punctuated correctly.

I actually prefer jwpat7's suggested rewording and punctuation.