The rules for drawing house districts really hampers the Texas GOP. You can't just connect Dallas, Harris and Travis counties with exurban/rural areas and create GOP districts that way. They have to stay within the county, which was fine when there were plenty of Republican leaning suburbs within those counties. Not the case anymore.
Looking at the Congressional maps, I imagine this only applies to the state maps?
That is correct.
Is that a state constitutional requirement or just statutory?
Sorry for the late response but here you go:
https://tlc.texas.gov/redist/requirements/house.html