Driving LED matrix with shift registers such as 74HC595

Wow, nice! That's one hell of a write-up!

You might find that the display is more averse to flicker if you do something like:

function r() { // run
  dnoe.reset();
  setInterval(function() {
    var rdx = 5;
    while (rdx) SPI1.write(dbuf[--rdx],dnss);      
    // whatever SPI write needed to turn all columns off
  }, 10);
}

By scanning all in one go and then turning off, you don't get a slightly brighter row appearing if some other JS code takes a while to execute.