Some (MeGUI\x264) MKVs playback as a thin horizontal line, apparently due to an error in the encoded display aspect ratio value. It seems the problem is not in the header, but in the x264 stream itself. The problem can be corrected by demuxing the MKVs and then remuxing them with MKVmerge, adding --engage remove_bitstream_ar_info in Extra options\User defined options of the x264 track - thus removing the incorrect display aspect ratio data.

I'm not sure how this figures in the whole, but in example A MKVmerge x26\Format specific options display width/height is reported as 1280160/720 (1280160 / 720 = 1778, the bad aspect ratio which MediaInfo reports for this MKV) and in example B MKVmerge lists width/height as 1279680/688 (1279680 / 688 = 1860, the bad aspect ratio which MediaInfo reports for this MKV).

For now, I'd be glad to be able to either prevent any A/R data from being encoded or specify it so it is encoded correctly and I'd welcome any suggestions on how to go about it.