As reported already elsewhere, I have massive problems downstream in my workflows with MKV containers created by TVAI .
With earlier versions of TVAI it was possible to preview MP4 and MOV while rendering. That has been renoved. The only container which allows preview with third party media players is MKV, currently. That’s the reason why I switched from MOV to MKV, but I’m very unhappy with it.
Does anyone know why preview while rendering with MP4/MOV containers has been dropped?
I think it happened during a change that was made because some users reported the preview supposedly overwrote their export file. I personally was not able to reproduce this during beta testing even though I set both the export path and the temporary path to the same location. And this issue never ever happened to me either. I strongly miss the ability to watch the export mp4 in progress too! 
Usually, with files created with regular MP4 and MOV muxers, both video formats cannot be played while written. That’s by design. Consequently, Topazlabs must have used some kind of fragmented MP4/MOV formats to allow playback wile rendering. Fragmented streams are more or less standard when streaming, but unusual on file based delivery and can create problems with some local media players or some NLEs. However, I cannot imagine that fragmented streams can cause overwriting existing files. That makes no sense to me.
Removing the feature must have been a deliberate design decision and nothing which is likely to happen by accident.
Yes, as far as I remember the processed file is written in chunks.
