DisplayPluginBase::getAttachedDisplays returns displays that are not attached.

Problem/Motivation

If we have a view with multiple displays for example:
Display 1
Display 2
Feed 1 (attached to display 1)
Feed 2 (attached to display 2)

The method getAttachedDisplays() when called on the display handler for Display 1 will return [Feed 1, Feed 2] instead of the expected [Feed 1].

Proposed resolution

// Go through all displays and search displays which link to this one.
    foreach ($this->view->storage->get('display') as $display_id => $display) {
      if (isset($display['display_options']['displays'])) {
        $displays = $display['display_options']['displays'];
        if (isset($displays[$current_display_id])) {
          $attached_displays[] = $display_id;
        }
      }
    }

if (isset($displays[$current_display_id])) needs an extra check for !empty($displays[$current_display_id]) because for the feed displays, the value $displays[$current_display_id] is set but is 0 if the feed is not attached.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet