It reloads the File page for me. Unless I'm fast enough to click the Play button again before the reload has finished as that does play the video for me.

I think this might be a Commons specific issue. If you disable gadgets with safemode=true, things seem to work fine. That would indicate that something is hooking into the link of the video button.

In T400106#11038202, @TheDJ wrote:

I think this might be a Commons specific issue. If you disable gadgets with safemode=true, things seem to work fine. That would indicate that something is hooking into the link of the video button.

I think you're on to something. If I go to a page on Meta-wiki with a video, set the ?uselang string, and play the video it works.

Example: https://meta.wikimedia.org/wiki/Wikimedia_Foundation/Communications/A_Wiki_Minute?uselang=nl

from ext.gadget.AnonymousI18N

if (conf.wgUserLanguage !== conf.wgContentLanguage) {
    $(document).on('click', 'a[href]', function(e) {
        var href_attr = $(this).attr('href')
          , href_full = this.href;
        if (href_attr.substr(0, 1) !== '#' && href_attr !== '' && href_full.substr(0, 4) === 'http' && href_full.indexOf('.wik') !== -1 && href_full.indexOf('.org') !== -1 && href_full.indexOf('Special:Set') === -1 && e.ctrlKey !== true && e.metaKey !== true) {
            e.preventDefault();
            location.href = AnonymousI18N.getUrlForLanguage(href_full, wgAnonUserLanguage);
        }
    });

We can probably use stopPropagation in the videoJS play handler, or Exclude <a role=button> from ext.gadget.AnonymousI18N

Edit request made at https://commons.wikimedia.org/wiki/MediaWiki_talk:Gadget-AnonymousI18N.js#Video_play