When installing a profile that doesn't sheep with the right configuration of seven theme for blocks, we end up with administration pages missing tabs.
One way to replicate the problem is to remove these files from profiles/standard:
- block.block.seven_breadcrumbs.yml
- block.block.seven_content.yml
- block.block.seven_help.yml
- block.block.seven_local_actions.yml
- block.block.seven_messages.yml
- block.block.seven_page_title.yml
- block.block.seven_primary_local_tasks.yml
- block.block.seven_secondary_local_tasks.yml
This will get the same result as installing any custom profile missing these files.
After installation, for example, in admin/contents, we have missing blocks for primary tabs:
It's missing the primary tabs (e.g. Files), the breadcrumb and the "Add content" button.
This is an old issue. Not sure if it was already reported and discussed here.