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.
All the other commits in this patch set are just refactoring/cleanups.
Targeting 5.25 branch, but the last commit (the actual bug fix) can be easily backported to %5.24.
Pretty sure there was a bug report about it.