Right now, EntityViewBuilder always uses $entity_type_id as #theme. #2698909: EntityViewBuilder uses non-existing #theme hooks → corrects that a bit and doesn't use it when no such template exists.
However, to view an entity, you still need to implement hook_theme(), define a template, provide a default template and if you want to do it like node, implement quite some preprocess logic, theme suggestions and so on.
This proposes to introduce a standard template, with the common preprocess stuff and theme suggestions.
Instead of node.html.twig, you would then use entity--node.html.twig and so on.
That removes all the unecessary glue code that is currently necessary to have useful entity templates.
Previous attempts at standardizing this failed ( #2023571: Support preprocessing in EntityViewBuilder → , #2270883: Automatically add theme hook suggestions for all entity types → ), this basically replaces those issues. Those issues were complicated because it affects existing templates, so there is a risk of changes and BC breaks.
None
A new template.
Needs work
11.0 🔥
The patch will have to be re-rolled with new suggestions/changes described in the comments in the issue.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Looks this approach is slated like #2270883: Automatically add theme hook suggestions for all entity types →
Additionally it needs to remove olivero_theme_suggestions_user_alter
https://git.drupalcode.org/project/drupal/-/blob/11.x/core/themes/oliver...