The distance fraction in the horizontal direction.
A value of -1.0 corresponds to the edge on the "start" side, which is the left side in TextDirection.ltr contexts and the right side in TextDirection.rtl contexts. A value of 1.0 corresponds to the opposite edge, the "end" side. Values are not limited to that range; values less than -1.0 represent positions beyond the start edge, and values greater than 1.0 represent positions beyond the end edge.
final double start