[jpsdr]

That'd be easy to do (ie. force thread mode to slice if DXVA is enabled in the video options), but not intuitive at all for the end user.

As ffmpeg ticket you point state something obvious : Either you do SW decoding (with mt), either you do HW decoding, but not both, it make no sense.

Unfortunately, non sliced sources will probably represent more than 99% of the files.

I think, as you said, the following should be done :
- If DXVA is enabled, force thread to slice, if DXVA is disabled, do not force thread to slice.

After, what i think is :
If someone's CPU is not powerfull enough to decode a 8bit h264 (even with mt), and must use GPU, it will never be able to play 10bit files, so for it, having finaly mt only on slice will have no effect.
If CPU is powerfull enough to play 10bit files (either without mt or only with mt), it can even more play 8bit files, so not having DXVA will finaly have no effect.

I will personnaly for now use only the mt version maruchan build (thanks), and probably stick to it and not even try nightlies if i don't see anything related to this point in the commits. Or, any updated build without the a0fc63d people may do.