Large Display: x by y w/ 24 bpp Graphics buffer visualized with neopixel string in zig-zag layout

I'd be pretty sure that this was actually an electrical issue due to the neopixels missing one or two bits (so one pixel's colour 'bleeds' into the next).

Are you sure the grounding is ok? And do you have an oscilloscope? Are the signals actually 5v?

Out of interest, what happens if you just do:

SPI2.setup({baud:3200000, mosi: npxPin});
SPI2.send4bit(g.buffer, 0b0001, 0b0011); 
// edited after @allObjects suggestions

Which is the 'old' style of doing neopixels.

I'm just aware that there was this PR 8 months ago that changed the waveform pattern: https://github.com/espruino/Espruino/pul­l/1459

There was obviously a lot of testing that went on but given it had been working fine for years beforehand, the phrase "if it ain't broke, don't fix it" could apply here.