Press the Mini timer button and the disk pops out into a small window that stays on top of your other apps. Keep a lecture video, a document or your slides full size and still see the time left.
You can start and pause right inside the small window. Close it and the timer keeps running on the original page.
The mini timer works in Chrome and Edge on desktop. In other browsers the button doesn’t appear — use the time left shown in the tab title and favicon, or the full screen timer, instead.