Most walkthroughs about browser-based rhythm tools focus on the musician's side: where to click, how to lay down a beat, what tempo sounds good. This piece flips the camera around and looks at the engineering surface instead — the grid data structure behind every row and column, the rules that turn a click into sound, and the edge cases that quietly break a prototype the moment it meets a second user. The lens here is the practitioner who has to ship, debug, or extend one of these widgets. Ev...