Does the em dash take precedence over the comma?

Question

In a sentence like:

While I would love to do A, I cannot wait to do B.

If I add a set-off clause right before the comma, which of these is correct?

While I would love to do A—for reasons X—I cannot wait to do B.

or

While I would love to do A—for reasons X, I cannot wait to do B.

How does one include a set-off clause right before the comma? Note that I wouldn't like to rephrase the sentence to avoid this dilemma. I tried looking for a solution in British English, but couldn't find a consistent rule.

Answer 1

Origin : While I would love to do A, I cannot wait to do B.

"While~do A" is an adverbial clause(an adverbial clause is also an adverb).

I cannot wait to do B while I would love to do A. We removed the comma.

It looks more easy now.

But you should remind that "while clause" almost always(as I know) is closely related with main clause so you need to take care when you input something.

Some general rules for em dash

1. Colon-like use : Simple equivalence (or near-equivalence) of colon and em dash

Three alkali metals are the usual substituents: sodium, potassium, and lithium.

Three alkali metals are the usual substituents—sodium, potassium, and lithium.

2. Inversion of the function of a colon

These are the colors of the flag: red, white, and blue.

Red, white, and blue—these are the colors of the flag.

3. Parenthesis-like use : Simple equivalence (or near-equivalence) of paired parenthetical marks

Three alkali metals (sodium, potassium, and lithium) are the usual substituents.

Three alkali metals—sodium, potassium, and lithium—are the usual substituents.

The food, which was delicious, reminded me of home.

The food—which was delicious—reminded me of home.

The food (which was delicious) reminded me of home.