I browsed my own Forum (XF version 1.0.4, default style, no add-ons installed) and discovered the following issue:
So I have a thread containing a YouTube-video and a text underneath that video in the very first post of a thread. The thread only consists of one post.
When going to a thread which contains a YouTube-video (the video is located at the very top in the first forum-post), then it is not possible to scroll-down to see the text which is posted underneath the embedded video.
I have also browsed some other threads of my forum which do not contain a video, and "scrolling down" in that thread was working fine.
Not sure, but could this be a bug somehow?