Not sure. I am going to have to pay attention when they occur.
I saw one, in the right bottom corner of my screen before I made the initial post.
Then, as soon as I finished with my initial post, I saw one on the left side of my screen. I took ruler and measure it - that one was about 9" W x 2.5" H. Most times they are more square.
I texted my brother. He asked what apps are running. and he mentioned about DOS shell.