Problem/Motivation
Once Classy extends from Stable it should not reference assets/templates/etc. from core modules.
Why this should be an RC target
Once we have all the templates and CSS library assets in Stable, anyone using Stable or Classy as a base theme should use the Stable or Classy namespace (depending) when they are extending templates or overriding libraries.
If we don't do this, then Classy and people working from Classy or Stable will be much more likely to extend core module templates and set themselves up for things breaking when we change those core templates later.
Bad:
{% extends "@block/block.html.twig" %}
Good:
{% extends "@stable/block/block.html.twig" %}
The changes required should be non-disruptive and are part of proper completion of the parent issue, which is tagged as rc target.
Proposed resolution
Update templates and .info.yml files. We can work on a combined patch while waiting for the parent.
We will need to ensure that a child theme can override library assets that a parent theme has overridden via libraries-override.
Remaining tasks
Patch
Review
User interface changes
n/a
API changes
n/a
Data model changes
n/a