The 99% Mystery: Why My ffmpeg.wasm App Stalls at the Finish Line

I’ve been building VideoSnap, a tool that processes video entirely in the browser using ffmpeg.wasm . For a long time, I was haunted by a specific, frustrating bug: the "99% Trap." A user uploads a file, the progress bar climbs smoothly, hits 99%... and then everything just stops. The UI becomes unresponsive. The fan starts spinning. It doesn't crash with an "Aw, Snap!" error, but it hangs there, sometimes for minutes. Then, suddenly, the download pops up as if nothing happened. I realized that