The reason this happens is because the composer title steals focus when the composer is opened.
discourse/composer.hbs at master · discourse/discourse · GitHub
So when you use the shortcut, you’re using it while the input is focused and that’s why it doesn’t work. You’d need to click somewhere outside to remove the focus from the input. Although slightly less obvious, the composer body text area - if focused - will also interfere with the shortcut.
When you enter full screen mode the title input is removed, when you exit, it gets re-inserted and steals focus again. This is why this occurs
Mousetrap has a way to handle this, to add a
mousetrap class to the markup of those elements, the shortcut would then fire even if the inputs are focused, but so will all the other keyboard shortcuts and it creates a lot more problems than it solves.
Sure, just let me know what works for you.