"Is this something that the writer decides, or is this dictated by the sentence structure?" The writer decides and uses the structure to convey the meaning clearly.

So, in the last sentence, say I intended "the upper left key" to be the noun. Then, the appositive, "the ` key", is restrictive so it shouldn't have commas. However, the lack of a comma seems strange. Second, say that I intended "the ` key" to be the noun, do you put commas around "the upper left key"?

That's a parenthetical expression, which once would have sat inside parens. To me, you need a later comma: "I use the upper left key, the ` key, to open and close Chrome."

I have a feeling that an appositive cannot precede a noun unless it's the very first thing that starts a sentence. And, if the appositive follows the noun and it's restrictive, I am guessing that you can use a comma if the situation requires once but you don't use the second comma.

@YosefBaskin In your sentence, you use a 2nd comma, but the appositive is restrictive. The upper-left key can be one of many.

I don't see it as restrictive since there is only such key. You are restating what key that one key is. My view, not arguing.

No: an appositive NP always follows the noun it modifies. no exception.

@YosefBaskin No, there are many upper-left-hand keys. You are changing the meaning of the sentence. I don't understand how it's your view when you can't see my keyboard. 😆

@BillJ What's an NP?

NP stands for noun phrase.

@BillJ Why do you say that the appositive always follows the noun? There are many instances online that state differently.

An appositive NP is a post-head modifier, not a pre-head modifier. Those on-line grammar sites that say otherwise are flat wrong.

@BillJ Doesn't the appositive precede the subject in the first sentence on this website? literaryterms.net/appositive

Sorry, but they are wrong. In their example 1, "a two-year-old rambunctious puppy" is not an appositive NP. It's an ascriptive NP functioning as a supplement. It's predicative in that it equates to "Brady is a two-year-old rambunctious puppy,

The rules for commas/no commas only apply when the appositive comes after the noun.

Unbaked, part of the confusion here is the understanding of what is “essential” to the sentence. A non-restrictive relative clause or appositive may communicate critical information, but if left out of the sentence, leaves the sentence grammatically complete.

Isn't a core feature of the contemporary definition of appositive that both must be able to serve as the subject of the verb and the two sentences should have the same referential meaning when the sentence is first made with one and then the other? Jane, a soprano in the church choir, had a cold and couldn't sing. is not an example of a sentence with appositive subjects, whereas Jane, the best soprano in the church choir, had a cold and couldn't sing is one.

It is not essential to know how to identify an appositive to write effectively or to punctuate effectively. The identification and analysis come later.

@Xanne: You don't have to know a lick of grammatical terminology to write well.

Appositive dependents are ones which when substituted for the matrix noun phrase in a declarative clause systematically yield a clause which is an entailment of the original: She sang in [the opera ‘Carmen’]. It was founded in [the year 1850]. [The verb ‘use’] is transitive. --> She sang in ‘Carmen’. It was founded in 1850. ‘Use’ is transitive. In each of these pairs, the first entails the second. The appositive thus provides a formulation that can stand instead of the NP containing it. (The Cambridge Grammar of the English Language p447)

@Xanne I agree that part of the confusion is what is essential. This is why I broke my question into parts. The answer to the 2nd question is contingent upon the 1st. Thinking about the 1st question helps answer the 2nd. I have a feeling that this is one of those areas in English which doesn't have a satisfactory answer. ☹