After choosing to remove the menu bar the only way i found to get it back was to remove the configuration files of gwenview... It would be better to have an option in the context menu for instance Reproducible: Always Steps to Reproduce: 1. Deselect Settings > Show Menubar 2. Try to get the menubar again.... [without CTRL+M, no human will remember all shortkey combination]
You can always hit ctrl+M to show the menu bar again. But I agree, this is a sub-optimal user experience. Duplicate of https://bugs.kde.org/show_bug.cgi?id=210620 *** This bug has been marked as a duplicate of bug 210620 ***