Just revert the code back to pre-26! This is ridiculous, it can't possibly be this hard and if it is, it just points to the degradation in the quality of Apple software! This is maddening!
This is already the pre-26 bounding box, isn't it? It's the new graphics that don't line up. (Not a great excuse, but the graphics are here to stay at least for a little while.)
I am using Sequoia and the windows are definitely rounded! Though the radius is pretty small (the curved region is about a quarter of the mouse cursor area), so the fact you can drag it from outside the window doesn't look ridiculous.