Three green tests locally, two red on CI. The outcome depends on how busy the build server is that day. This isn't a logic bug — it's a time.Sleep that assumes 100 ms is always enough for a goroutine to finish.
That test doesn't verify anything. It gambles.
testing/synctest, stable since Go 1.25 (August 2025), solves this at the root. No more guesswork, no more arbitrary waits: the clock advances on demand, inside an isolated bubble.
The test that lies
Debounce is a classic example of time-dependent concurrent code: trigger an action only if no call has happened in the last N milliseconds. Here's a simple implementation.
