White rectangle obscuring most of the screen

Apple must devote a lot of time thinking of ways to annoy me. The latest problem is a big white rectangle which covers most of the screen and won't let me get rid of it. Sometimes I can do so by exiting Safari and then going back in. Sometimes I can only get rid of it by shutting down the computer and restarting which is time consuming. Any idea as to why this happens and how can I resolve it? I Haven't got an old clapped out computer which is on its last legs. I'm using an M1 Mac mini which I purchased last year so there shouldn't be anything wrong with it. I'm running Sonoma and the operating system is up to date