borderWidth property
The width of this side of the border, in logical pixels.
Setting width to 0.0 will result in a hairline border. This means that the border will have the width of one physical pixel. Hairline rendering takes shortcuts when the path overlaps a pixel more than once. This means that it will render faster than otherwise, but it might double-hit pixels, giving it a slightly darker/lighter result.
To omit the border entirely, set the borderStyle to BorderStyle.none.
Implementation
final double? borderWidth;