Having the same issue as some before me. The Snapshot function is absolutely great to have but it seldomly works properly. It usually cuts off parts of the right and the bottom. This renders the desired function of screenshotting only a part of the screen useless. Anyone found a solution for this problem?
related thread with an older version of opera: https://forums.opera.com/topic/22896/opera-48-0-2685-50-snapshot-tool-not-taking-desired-snapshot?_=1581669003349