"situation differs from state to state"

Now that I had time to do some research, this is a reference for partial year MD residency:

https://www.marylandtaxes.gov/forms/Personal_Tax_Tips/tip52.pdf

"Complete the name and address information at the top of Form 502."

"Place a "P" in the box and show the dates of residence in Maryland"

"Enter on line 1 the adjusted gross income from your federal return."

"Include on line 13 any income received during the part of the year when you were not a resident of Maryland."

In other words, you don't change the W2. Adjustments are accounted for elsewhere.

And you might even have a taxpayer with multiple pages of W2 all from the same employer, because that is how payroll would break out the reportable amounts by State. That's why you don't add them up. There will be States where the full Fed amount is also reported to that State.