Created attachment 132439 [details]
Screenshot showing the mirroring tool effects for different brushes

SUMMARY
Mirroring tool does allow setting the mirroring line in the middle of the pixel, however it doesn't work with even half-pixel precision.


STEPS TO REPRODUCE
1. Make a new document 21x21 pixels. 
2. Turn on mirroring.
3. Try to paint. (You can use Pixel Art brushes for that; other engines give the same results though, except for Quick Brush engine, which would *WORK* with half-pixel precision but would move the mirror line one pixel later).

OBSERVED RESULT
The mirroring line is in the middle of 11th pixel, but it behaves as if the line was between 11th and 12th pixels.
For example if you paint 9th pixel, then the 12th pixel will be painted on the other side.
If you paint 10th pixel, then the 11th pixel will be painted.

EXPECTED RESULT
Mirroring working fine even with the mirror line in the middle of the pixel.
For example if you paint 9th pixel, then the 11th pixel should be painted.
If you paint 10th pixel, then nothing should happen because the mirrored pixel of 10th pixel is the same pixel (10th).

ADDITIONAL INFORMATION
I showed the effects in the attachement.
I used those brushes:
- "Pixel Art" for Pixel Art brush,
- "Ink-1 Precision" for Pixel Engine brush,
- "Basic-1" for Quick Engine brush.


SOFTWARE/OS VERSIONS
Krita

 Version: 4.4.0
 Languages: en_US, en, en_US, en, en_US, en, pl_PL, pl, pl_PL, pl
 Hidpi: true

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9