Articles

The Snail Spins Around the Instant You Tap: One Frame of Clock Skew, Amplified by a Modulo Into the Worst Possible Error

A mascot scene card driven by SwiftUI's TimelineView had two defects: the snail mirror-flipped in place the first time you started a sound, and the background hard-cut when you stopped it. The first one took two rounds to bottom out — round one blamed a paused timeline desyncing the two clocks, and the flip survived the fix. The actual cause was one line of phase normalization, `raw < 0 ? raw + 1 : raw`, which took the few-dozen-millisecond fact that timeline.date trails Date() by a frame and wrapped it into a phase of 0.9999 — and facing direction happens to be a discontinuous function of phase at zero. This postmortem covers why .transition doesn't work inside TimelineView, how to express animation state as a pure function of time, and why a crossfade should be fade-in only, with no fade-out.