@MichaelSchoenitzer Was this on wikipedia ? Considering that mini is not a standard file syntax specifier....

Nothing to do with the page-previews popups.

Yes, this was on de-wikipedia: https://de.wikipedia.org/wiki/Benutzer:MichaelSchoenitzer/test

'mini' is the german version of 'thumb'.

Hmm, it's working ok for me in Chrome, Firefox, and Safari on your test page and a copy I made here: https://de.wikipedia.org/wiki/Benutzer:Brion_Vibber_(WMF)/Test

Might be a compat issue with a user script or gadget, or another config setting maybe...

Can confirm that the issue seems not to appear with a brand new account. It does however occur on both, my private and my work-account. Since I don't have much customization in later it's likely to be something widespread. :/

Thanks for the confirmation, I'll see if I can figure out which option is troublesome...

@MichaelSchoenitzer looks like you have 'use live preview' on (labeled in English as "Show previews without reloading the page" in prefs editing tab). I can confirm that the player fails to work in the preview in this case -- it's probably not loading the required JS module -- but I haven't been able to repro the visual offset issue yet, at least in Chrome.

The JS should be fixable, but if the styling is off regularly I'll want ot track that down too if possible. Can you check in "inspect element" on the <video> or the surrounding spans if there's anything suspicious on the alignment? I'll try to track down where it came from, but there's a lot more potential sources in your site JS so hoping for any clues that might be hiding in there. ;)