Once starting, if you’re in flow, just keep going.
Starting is usually the hardest part for me, so the pomodoro method is a nightmare, because it greats more start times. Not forcing the break if things are going well helps.
Having the window go black would drive me insane. If I’m on a train of thought and my screen blacks out, I would need a notebook to frantically write down where my head was at and what I was doing when it went black. Then I’d spend the first 10 minutes of the next block trying to get back to where I was mentally.
I thought the idea of an hourglass would be better, to try and work for at least X minutes. If things are going well, the hourglass silently ends and I could keep going. If I’m having trouble, I could see the hourglass is finished and take a break, then try again. I tried this briefly, but the sand kept getting stuck, which I found problematic. I supposed pomodoro software without an alarm would do the same thing.