What exactly "long" means depends heavily on the machine that is running Cockpit as well as the bitrate/resolution of the video.

Usually 1080p videos can go up to 80 minutes, while 4K ones are usually more in the 30 minutes range.

This is a critical bug, since it's one of the main functionalities in Cockpit, and the user usually discovers the problem while in a mission, or after one, and seeing the result of several hours of work to "fail processing" can cause some serious anxiety.

It's not yet clear to me how to address that. Maybe we can write the chunks to the merged file without actually opening it in memory? Or maybe the solution is to try processing the video, and if it fails, divide the processing into smaller sections that can be processed, and deliver N video sections instead of a single file? I don't know, but the current situation is not good.