Views "display menu tabs" styling breaks when adding an "Entity Reference" display

misterdidi → changed the visibility of the branch 3457768-views-fieldset-labels to hidden.

misterdidi → changed the visibility of the branch 3457768-views-fieldset-labels to active.

After digging into the code, I found out that :
- Claro module moved its file responsible for the styling of tabs css/components/views-ui.css from global-styling library to a new one called views_ui which extends views/views.module library.
- views/views.module is attached to any new ViewExecutable object through its $element property and used in the render array of Drupal\views\Plugin\views\display\DisplayPluginBase::render().
- for Entity Reference display, the display plugin Drupal\views\Plugin\views\display\EntityReference overrides its parent render() function and does not attach the view $element property to the render array. Hence, views/views.module is never attached and the claro/views_ui never used for an Entity Reference display.

I wrote a patch in which I chose to alter the render() method of EntityReference to mimic DisplayPluginBase::render() and always return the view $element['#attached'] value with the render array.

I believe this issue is a part of 🐛 Secondary tabs design is distorted on block view page Active .

If it really is, I believe this is a duplicate.

@misterdidi can you confirm if this is a duplicate? If not it should go back to Needs work though as will need an MR to review.

Confirmed: This is a duplicate of 🐛 Secondary tabs design is distorted on block view page Active . The problem there (view-ui.css is missing) is the cause here, too, and adding it fixes the problem in both cases.