I only had one tab open anyway. But, what would you know, tapping on the square in the Browser bottom bar did not bring the list of tabs as expected. Oh no. It brought the unlock screen where you need to tap your PIN. If I understand it correctly, my screen timed out while I was watching the video but the video playback prevented it. But some flag was set somewhere that the screen had timed out and so the unlock screen was shown at the nearest opportunity.