As mentioned in comments on #43646, a negative channel value in preview
view settings can be used to climb the metastack. But the functionality
did not work as expected in most cases.
This combines a one-line fix by Sergey with an updated description for the
property which documents the feature.
In the future we may want to change its operation to be less obscure,
since it has some use-cases in aligning strips within a metastrip with
those outside of it.
1cb30b9d1e