I tested the clip studio paint’s close gap feature. They have a setting where you can specify the amount of gap by pixel. So 50 100 px value covers most of the gaps. It fills the whole area but sometimes it doesn’t go to all the corner so there is some amount of work not much but it is there and it is expected.
A test with your image provided in source
As you can see the gap width determines the extent of fill. Setting the gap width high it neglects the hair outline and fills even the face. But here too at some point you need to click multiple times like the ponytail is not filled at once.
The flood fill option in your test app gave me instant complete fill although it left edges of white border.
And lastly I tested mypaint with same image.
I think mypaint beats all the software I tested, although it has slight lag it fills very well as expected even the ponytail and you can control how much aggressive it should fill. I believe we should take a look at mypaints gap filling code which can be found here - mypaint/lib/fill at master · mypaint/mypaint · GitHub