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 the 99% mark isn't where FFmpeg is working—it's where the browser is fighting for its life. The 99% isn't FFmpeg—it's the Handover In ffmpeg.wasm , the progress bar tracks the FFmpeg execution. When it hits 99%, the heavy lifting of transcoding is actually done. The "hang" happens during the handover: when you call engine.readFile() to pull the processed video out of the WebAssembly virtual memory (MEMFS) and into the JavaScript heap.…