After completing of #2 one more defect was found.

The offeset between vertical lines from parent and child blocks can be 1px, but always should be zero.
Possible reason: the calculation of the offset wrongs if the width is not an even number since left: 50% can be +- 1 px.
Acceptance criteria: confirm that my proposual about the reason is correct and fix it.
After completing of #2 one more defect was found.

The offeset between vertical lines from parent and child blocks can be 1px, but always should be zero.
Possible reason: the calculation of the offset wrongs if the width is not an even number since
left: 50%can be +- 1 px.Acceptance criteria: confirm that my proposual about the reason is correct and fix it.