Hello friends of Krita. This is my first message in this forum. I'm looking forward to contribute here, to ask questions and maybe give some answers at some point.

My current problem is that pixels outside the selection are used by the Blender_Blur tool as a source for the blurring process. I would like to use only pixels within the active selection as source and target for the blur process. The tool should not work outside of the selection at all, neither read nor write. Is there a switch or another way to do this? Could this be a new feature or does it already exist (still using Krita 4.1.5)?

Please first check the latest version of Krita, 4.2.7.

Thank you very much for the quick reaction. I have now installed 4.2.7 and was impressed how smoothly this went. As far as I could see, all settings were preserved.
Back to the topic. The behavior of the Blender_Blur tool has unfortunately not changed. Any idea?

I recall this being reported, but I can't find the bug...

Either way it'll take a small forever till it's fixed, last time it was discussed there was no easy anwer; the problem is that we somehow need to consider everything outside the selection the exact same as the edge.

Isn't it enough to ignore the pixels outside the selection?

No, because then there's a chance that the pixels outside the selection will be interpreted as fully transparent black pixels, and I don't think that is your intention? So we need to modify the whole thing to understand how to interpret pixels outside the selection.

That really depends, behaving as if the unselected part of the layer was empty is often (well, actually most of the time) just what I want, otherwise I'd have to create a new layer every time. But people's workflow may vary of course, some convenient option like e.g. the lock alpha would be really useful in that case.

But I also wondered if those brush engines could just work on less pixels in that case instead of assuming any particular value for them, but I'd have to look at the actual implementation...it would of course affect the strength of the blur effect near selection borders, but as option it could be useful.

To explain what kind of behavior would seem consistent to me. The Blur tool has a circular imprint. Assuming it would exceed the straight border of a selection by half, the impression should have the shape of a semicircle. Only pixels within the selection may be smeared.