Amends c067d498.

Having `preferredWidth: 1` set on an only item that also stretches with
fillWidth AND without top-to-bottom sizing (fixed size imposed by
parent component) doesn't really make sense. It naturally increased row
layout's implicitWidth by only 1, and that value were used as an input
to the final width expression (bottom-up sizing).

This patch fixes the sizing model to be strictly implicit bottom-up and
explicit top-down, and so the slider no longer collapses.

BUG: 446185