This is not really that odd... the xine Pat packaged into slackware is about as outdated as it could be. Mplayer should play most of your videos while the packaged xine will likely give you a screen looking more like a screen saver or a bad acid trip.

This was my solution. I compiled ffmpeg from slackbuilds.org, and upgraded xine myself. My build scripts are here:
https://github.com/WhiteWolf1776/slackbuilds

This allows xine to play about anything I wish. It even had the side benefit of getting bluray working if done correctly.

I tried xine-lib-1.2.4 a while back when I first encountered this and if anything it was actually worse than the 1.1.x in slackware, but I see there were significant changes in 1.2.5, so maybe its time to have another try (1.2.4 was the latest available when I last tried to build this).

Yea, 1.2.4 had several issues, my personal fav was... it picked a random audio track and would not change... ever... guess it wasn't completely random, but if you had a vid with say several languages.. get ready for a good party game of 'guess that language'.

So far, 1.2.5 is smooth, works, and has made my life rather easy... the xine part anyway.

That is indeed a pity. I think i3 in SlackBuilds may also be a bit behind the curve. i3 worked perfectly on Debian, but the package from SlackBuilds had a couple issues. At least for me. Whether the problem was i3 or me ...? For now that is an unanswered question.

Thanks for the link. I shall not be experimenting with the packages there, until after I try getting Ibus installed. Unfortunately, that is a deal-breaker for me. If I can figure out how to install it, then your xine package will get a go. It looks interesting. I am waiting until I get the motivation to re-read the package management section of the Slackbook. (Making and/or installing packages from outside sources is still new to me. It will take a while to wrap my head around it.)

i3 on SBo is at version 4.8, which is the last stable version. You may want to open a new thread about that, maybe we can help.