I was going through some of my video files just now and was using the super handy viewer that I love oh so much. Suddenly, I get a mountain of green text and gobbledigook in the viewer when I selected an MKV file. Does v13 not support MKV video files or have I screwed up my settings somehow? I am 99.99% certain that the viewer in v12 played MKV without issue. Do I have to get a special codec or copy one to the new install directory? Just curious.
Thanks guys and gals!
Works fine here.
Download K-Lite Codec Pack (codecguide.com)
Seems to be more a format than a container problem. AVC files play well, HEVC do not. Applies to .mov and .mkv containers.
Suggested codecs are listed near the top of here:
Those will work with newer playback APIs. Codec packs like K-Lite and LAV Filters will also work with the older DirectShow API, which we also support, but I'd try the ones listed there first, as they're usually all you need, and get updated automatically once installed.
@lxp I believe this has something significant to do with it. I was trying to use the viewer last night with an MP4 and it gave me same response as here, but played normally in VLC. I just did some digging and discovered that all the files share the same video codec - HEVC. For some reason, upgrading to v13 has removed DO's ability to play these files. I really, really do not like installing codec packs for all the excess garbage it puts on your system and screwing with your defaults, but if that is the absolute ONLY way to recover this functionality, I suppose I will have to.
Is that the only way to get DO 13 to cooperate with HEVC? K-Lite codec pack?
You just need to install the things I linked to.
I previously already had the MS Store version installed. Now it won't let me reinstall it. And I am reading that the .99 one doesn't work from the reviews im reading on its page. Plus **** paying MS for a damn codec. Theres no other way to get it?
I just installed the LAV filters - they work fine, go for it!
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@lxp thank you for your help man!
LAV Filters were also in the list I linked to. 
They're good if you need the one MS codec that isn't always free and don't want to pay the dollar. (It's going to the patent holders rather than MS, I think, although MS are probably one of the patent holders. No one seems to know for sure when/why it is/isn't free for some people.)
LAV can also be good for some older, more obscure formats as well.