Not sure what you mean by container repacking, but if you mean "Converting to supported codec", you can try instead from the File menu to convert using the "Fast: Full quality remux (no audio), likely to fail"-method. that is much faster and if it works it allows native scrubbing. Also I'm planning on improving the "Unsupported videos" experience in the future by using something like the MediaStream API

>> by container repacking losslesscut converts h264 streams recorded with streamlink from twitch, also it tell that video is unsupported.

>>Fast: Full quality remux (no audio) is audio really removed? can it be added with other editor?

>>MediaStream API on windows, what about microsoft media foundation, chromium has it in early stages