Nany 2018 idea.
A sand clock.
Configurable count back time. For example 25 minutes.
When the count finish give an auditive warning . But don't lose the focus. Even is configuration consist in activate a warning window or emerging window. The sound or warning configurable.
Each time after that, if the user don't abandon pc work repeat the alarm sound every time, or the visual warning. For example if you configurate repeat the alarm every five minutes, the new alarm will sound every five minutes.
When I come back to the pc the program is able to recognize real keypressing or mouse movings. Then the time will began to count down again. The clock will put itself down to up again.
I haven't seen this in all the pommodoro technique software. I think this is simpler and give freedom in the rest period. The important thing is abandon the pc for a while and fight the sedentary life.