I reckon Guisborough is where proper North Yorkshire starts, anything north of GUIs/Roseberry Topping is North East, not sure that’s factual because they’re Redcar & Cleveland but around that area they’re all wholly backs.
Middlesbrough is further North than Darlington.
Middlesbrough isn’t anything to do with Yorkshire other than whatever ceremonial purposes means, if you drive A19 south towards Thirsk you’ll pass a North Yorkshire sign.
We share news/radio/bus routes etc with yourselves, sounds daft but North East football awards always between 3 big clubs, Yorkshire have their own, everything about our town points North, where maybe well before my time it was culturally Yorkshire. If you asked 100 people in Boro (under 40) where they’re from they’d say Teesside or North East with the odd one or two saying Cleveland which stopped in 96.
When we go on Holiday constantly get mistaken for a Geordie or Mackem despite clear differences.