While adjusting the alignment for children (adjustAlignmentForChild()), we should bias offset the delta when center positioning.
adjustAlignmentForChild(*child, availableAlignmentSpaceForChild(lineCrossAxisExtent, *child) / 2);
Check FF behavior, but prefer subpixel positioning.
These custom controls look different on FF. By looking at the vertical positioning, the old behavior looks more centered. This is similar to bug 101848 (widget content centering)