well I had need for it so I made it, I forget to eat loads of times because of work and this will help me go eat on time. Eating on time is cool I think even though I don’t do it.
But to answer some points.
The stopwatch is not meant to measure the time doing the work but the amount of time your in front of the screen , you can pause it anytime like normal.
As you said Krita has a internal timer that updates upon saving. On the info tab, it exposes the xml values of the *.kra of exactley that, so when you save you get the Active Time of work of that given piece (in human readable time) if the file was not reset you will have the values since creation. And it will display the name of the original creator too if possible. Worth to note that really old files don’t have the xml with the same organization so sometimes it wont load up properly or at all (but if you know where the number is you can tweak the code for it, I left the current entries that i did not use on the code so you can read from where stuff is comming inside the code and not just some random numbers out of sequence).
long story short you can see the total editing time of a piece just as you can see from document info.