How is title case applied to idioms containing prepositions?

Question

For title case that does not capitalize articles, conjunctions, and small prepositions, how should one capitalize compound verbs and idiomatic phrases containing one of these elements?

For example,

when in doubt, smash it

Following the title case I described, it could be written as:

When in Doubt, Smash It

However, the "in" is part of the idiom "in doubt" rather than being used alone. Using that logic, it might be more appropriate to write it as:

When In Doubt, Smash It

Which way is most appropriate? Do any style guides address this problem or similar ones?

Answer 1

The various style guides that I'm familiar don't alter their guidelines for title capitalization depending on whether an included preposition (such as in) is part of an idiom or not.

That's not to say that none of them ever alter their preferences in special instances—such as when the preposition is more closely associated with the verb than with the associated prepositional phrase, as discussed in earlier Q&A's on this site (How to capitalize "get out of" in a title according to CMOS? and Capitalising "for" depending on the usage in the title?).

Style guides do the darnedest things, so I wouldn't be stunned to learn that some guide somewhere does treat prepositions in idiomatic phrases differently from other prepositions playing the same role but not in idiomatic phrases. From a reader's perspective, though, I don't see any advantage to be gained from making such a distinction, which is probably why most style guides do not endorse treating prepositions in idioms as a special case.