Well all they needed to do was erase the screen with white and draw on it, but their app's internal logic meant that they erased it more than once.

I was capturing QuickDraw library calls - the low level graphics primitives, to figure out where the graphics time in apps was going and found out sometimes excel did it 9 times

Of course users didn't see it more than once, but our hardware made all that wasted time run faster

Several layers of white is what makes the black really pop. (Just kidding).

It’s necessary for erasing cat pixels.

I think it could call (their equivalent of) clearRect up to 9 times on an already cleared region before drawing there?

It means they were time travellers! Secretly, they came from an alternate future where everyone used e-ink displays, and wanted Excel to be ready!

before writing to some area, it would erase it (clearing with white) up to 9 times