I write $2006 at the first PPU scanline end,
so the loopy_v changes.
Then write $2007.
But now the loopy_v has been modified by the scanline hardware.
For example
sta $2006
sta $2006
(loopy_v = $3f00, now the scanline end , loopy_v += ***)
stx $2007 (it's not the address which I want to write now)
so the loopy_v changes.
Then write $2007.
But now the loopy_v has been modified by the scanline hardware.
For example
sta $2006
sta $2006
(loopy_v = $3f00, now the scanline end , loopy_v += ***)
stx $2007 (it's not the address which I want to write now)