Updated: Comment #0
Problem/Motivation
Over in #504962: Provide a compound form element with accessible labels
@effulgentsia removed support for #value attribute of #type = details, as we already have #children.
However, this broke a few modules including views ui, so there are places in core that use #value with #type details.
Proposed resolution
Remove #value support from #type details, child elements should be just that... child elements.
Fix instances in core that use #value.
Remaining tasks
Patch
Tests already exist so we will get immediate feedback.
User interface changes
None
API changes
No longer support #value for #type details, although arguably this should not have existed in the first place.