A Chinese technical site cited QUAD7SHIFT as a flicker-free reference implementation. I didn't design it to solve flicker. I just built it correctly.

The Problem Nobody Talks About

Search for "74HC595 7-segment display Arduino" and you'll find dozens of tutorials. Most of them work — in the sense that the display shows numbers. But watch closely under fluorescent lighting, or point a camera at it, and you'll see it: a faint but persistent flicker, sometimes a ghost of the previous digit bleeding into the next.

This isn't a power supply issue. It isn't a loose connection.…