I’ve tried it and confirmed that the (+) button works when running in safe-mode and only the top tick box (Disable themes and theme components) is selected.
So we’ve got a dodgy theme and/or theme components.
I’ve gone through each theme, and they all have the same problem. So it’s something common to all of our themes.
Also, I’ve noticed that when you click the avatar, search, or hamburger, in the top right then it produces a drop-down and that THIS drop down is BEHIND this reply window, which isn’t true on meta.discourse. So it might not be that the (+) is behind, it might be that this reply window is forcibly on-top of everything.
Well, on meta you have a sidebar hamburger. You also have a chat window, and this chat window appears behind the reply box. (and the placement of it is terrible as it’s just slightly off centre?)
Hopefully whoever made our themes can investigate and figure out the conflict.