You say 'offensive', I say 'delightful'...
This is a new option in 18 related to rendering that somehow got turned on for the project you're working on. The US template default is 'off'.
It's called the "Render Safe Frame" which allows you to see what part of the 3D window will be rendered. We've never been able to see that before...resulting in trial and error renders, or post-render cropping in an editor. So, it's a great thing! Except for when you don't want to see it, like now.
😉
There's an on/off button in the "3D Visualization" toolbar... or menu: Document > Creative Imagery > Render Safe Frame
There is also a checkbox for the same feature in the Size panel of the Rendering Settings dialog. So, 3 places where it can be turned on/off... and the setting is memorized as part of views AFAIK.
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB