How to align the non-positioned children in the stack.
The non-positioned children are placed relative to each other such that the points determined by alignment are co-located. For example, if the alignment is FractionalOffset.topLeft, then the top left corner of each non-positioned child will be located at the same global coordinate.
Source
FractionalOffset get alignment => _alignment;
Source
set alignment (FractionalOffset value) {
if (_alignment != value) {
_alignment = value;
markNeedsLayout();
}
}