The bug on fullscreen mode depends on the position of the video
For example, on Elite:Dangerous topic, the fullscreen mode works for the video posted in the 14th post, but the mode doesn't work for the video in the 36th post...
Maybe a link with the scrolling, and the dynamic load of posts..
Not depending on navigator, and not linked to the net requests closed (appearing in console) and due tu mixed content (http content (like imgur links in E:D topic) on an https site)