Bugfix/15161 nested elements and error summary

[i-just]

Description

When attempting to save the owner element, if there are errors in the nested elements inside a matrix field, show a top-level error for that field on the owner edit page.

This should happen for cards and element index view modes, and the inline-editable blocks should continue to show the nested entries errors.

Additionally, if you open the nested entry in a slideout, it should be prevalidated and show the errors straight away, without having to save first.

This is a draft for now - to be discussed in the next meeting.

Related issues

• #15161