Changing buffer changes display area

[Paul]

I have two buffers open on two different files. In buffer 1, I go to the bottom with G, and zz to put the cursor into the middle, and see the ~'s in the bottom half of the window. I swap to buffer 2 with :bn. I swap back with :bp. Vim has 'optimised' the display area by moving the bottom line of the file to the bottom of the window. I would like it to keep it in the middle, or wherever I had put it. Is it possible?

When I search for this online, I see only results for moving the cursor position with startofline, nothing about keeping the displayed area as it was.