Only preview image of the video from a Mastodon server.
Single way to see ever the video is to use the "window in window" or "frame in frame" square button at top of the video preview. And it opens in a small rectangle.
For comparison, in the Social Web of ActivityPub plugin, after clicking at the image of the post, the post with shown in the separate section, but at least it shown fully, it is possible to open that video in that section...
Are related fix included into coming release? Or it will be addressed later? (at least I hope a fix for this is planned...)