I just discovered this odd issue in Safari 12 under macOS Mojave (10.14). It might exist in older versions of macOS but I didn’t start using full-screen—and Mission Control—in earnest until around macOS Mojave (10.14). And it’s reproducible from my experience across multiple macOS installs between my personal machines, the Apple store machines and even on my work computer.
Open up Safari, click the green dot in the upper left-hand corner to make this screen fullscreen and then hit the key combo for a new window in full-screen mode: Option+Command+N. Nothing happens.
Now move your mouse over to the “File” menu in Safari and just toggle it. With that done, hit the key combo for a new window in full-screen mode again: Option+Command+N. It works! Not only that, it works from that moment forward without any toggling of the “File” menu dance.
Is this a bug or a feature? If it’s a bug, how can I report it to Apple? If it’s a feature… Why is this desired functionality?