Opera in fullscreen mode preserves space for OS menu bar when switching to it from a desktop, demonstrated in attached animation. When switching from another fullscreen application everything works fine.
Issue appeared after last update
Version: 51.0.2830.40 - Opera is up to date
Update stream: Stable
System: Mac OS X 10.13.3 64-bit
Screen recording of an issue: https://www.dropbox.com/s/0q46pej6lxem6yj/menubar_issue.mov?dl=0