It's more commonly heard in the north and it's more of a dialect/regional thing rather than correct English.
I disagree. It's perfectly correct, and you hear it everywhere. I live on the south coast and people say it all the time.
There are several other examples of variations in verb tenses between American and British speakers. It's not a question of correct vs incorrect, but rather of focus. If you want to communicate an action in progress, you use present continuous. If you want to communicate a physical position, you use an adjective. Both are grammatically correct.
I could bore you with some other examples if you're still not convinced.
Agreed, my son says "the thing what I bought" and it drives my mother crazy! It's a northern thing.
This is an entirely different thing. Here "what" is used non-standardly as a relative pronoun. So in this case it is a regional variation that is considered incorrect in standard English, it is not a choice between two standard forms as in the previous example.