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It seems I made a mistake. I assumed it was the video tag that was causing the issue. After further checking, I see that the video tag is on other videos on the site and some of them are also set to 0 but are still able to go fullscreen and return properly. See the attached video below:
Do you still want me to zip the theme I'm using?
Something sets wpForo videos width=0 and height=0 and that function adds the data-orig attributes. It's obvious that the wpForo videos are affected by something. We've nothing to fix in the plugin for sure, we just need to find the problem maker and try to prevent it doing that. So please attach the theme you use and keep disabled other plugins during the test. Make sure the data-orig attributes are not added by other plugins.
You can find the video tag at twentytwenty/assets/js/index.js.
Insert this CSS code in Custom CSS Code Field from Dashboard > Forums > Settings > Styles Tab
Don't forget to clean all caches and refresh the webpage by twice clicking CTRL + F5
#wpforo #wpforo-wrap .wpfa-item.wpfa-video{
min-width: 300px;
}