I looked a bit into this, this would require digging a lot more to pinpoint where exactly is the javascript code auto-starting the video playback.
For now, one could make use of the new “Block media elements larger than […]” setting. For example, if I enable the blocking of media elements larger than 1000 kB (1 MB), the video won’t play.
However, the issue I find with the specific case you provide is that I cannot easily make the video start, because of the way the web page is designed. For now what will work is to select “Temporarily Allow Large Media Elements” in the context menu, then press the play button. Unfortunately this will cause all large media elements to be allowed for the site as long as the tab is not closed.
I will investigate to make the load-on-demand code smarter to be able to handle such case, i.e. to make uBO able to detect the media element under the mouse pointer even if that media element is covered by other DOM elements – which is the case in your specific example.