Everybody has asked for one. Nobody ships one. The reason is not taste, it is 15 millimeters.

Scroll wheels on current mice run roughly 15–22 mm in diameter. A thin laptop's keyboard section gives you about 10 mm of internal height, total. You cannot put a 15 mm wheel in a 10 mm space. And shrinking the wheel below 15 mm doesn't rescue you: it gets unpleasant to use, and you still have to redesign and miniaturize the optical encoder, the bearing, and the ratchet to fit.

So instead the industry gave us the scroll strip painted onto a corner of the touchpad. Those are too sensitive to be precise, they break after driver updates because they share the touchpad's driver stack, they eat usable touchpad area, and they make you steer around a region of your own trackpad to avoid scrolling by accident. Most of all, they feel like dragging a scroll-bar thumb. They do not feel like a wheel, because your fingertip has nothing curved to register against.

Arrow keys are the other fallback, and they remain terrible. In a word processor they crawl line by line and then lurch a full page.

This project's answer is in the Details tab in full, with a stack budget that lands at about 9.5 mm — inside the 10 mm envelope. The rest of these logs walk through how, one piece at a time.

If you think the 15 mm number is wrong, or that there's a mechanical wheel design I've missed, please argue with me in the comments. That's the most useful thing that could happen to this project.