Let me introduce you to a little something called the Potomac River—which is in the North, because Virginia's state line doesn't start till the far riverbank, suck it, traitors
Yeah, it's complicated because Maryland *was* Southern, if always somewhat odd because of its Christian pluralism, but the rise of Baltimore as a commercial port & the state's population boom 1840-60 unsouthernized it & aligned the state with Northern democracy & interests just in time for the war.
you have weird little exceptions like Texas Germans and Louisiana Isleños but if a state was outside the South it was flooded with immigration from Europe for some part of the Ellis Island era
Texas is weird because it's big enough to both be a marginally Southern state & be an unquestionably Southern state simultaneously. It's like asking if Indiohiokentuckennessee is a Southern state.
Texas is a Southern state and a Southwestern state, just as Pennsylvania is a Eastern state and a Midwestern state. Or Kansas being Midwest/Great Plains.