Nested slots value when multiple sources

In UI Patterns 1.x and UI Patterns 2.x, we can put many sources inside a single slot. Each source is a renderable and the slot is a list of those renderables.

If I put a single WysiwygWidget, I have a single renderable:

[
  "#type" => "processed_text"
  "#text" => "<p>Hello</p>"
  "#format" => "basic_html"
]

Because of this rule in ComponentElementBuilder::buildSlot():

    if (count($build["#slots"][$slot_id]) === 1) {
      $build["#slots"][$slot_id] = $build["#slots"][$slot_id][0];
    }

If I put 2 WysiwygWidget, I have a list of 2 renderables, of for each source:

 [
  [
    "#type" => "processed_text"
    "#text" => "<p>Hello</p>"
    "#format" => "basic_html"
  ],
  [
    "#type" => "processed_text"
    "#text" => "<p>World</p>"
    "#format" => "basic_html"
  ]
]

Problem/Motivation

However, sometimes, a source renderable is itself a list of renderables.

For example, in a view, with a WysiwygWidget and ViewRowsSource, I have nested lists of renderables:

[
  [
    "#type" => "processed_text"
    "#text" => "Hello"
    "#format" => "basic_html"
  ],
  [
    [
      #theme" => "node",
      "#node" => Drupal\node\Entity\Node {},
      "#view_mode" => "teaser"
    ],
    [
      #theme" => "node",
      "#node" => Drupal\node\Entity\Node {},
      "#view_mode" => "teaser"
    ]
  ]
]

It may not work well with component template looping on a slot and expecting a single level:

      {% for tab in items %}
        <li role="presentation">
          {{ tab }}
        </li>
      {% endfor %}

Proposed resolution

1. Pierre's: a configless automatic rule or nothing

For example, flatten the slot values, only first level, only when multiple sources, only when one a source renderable is a list of renderable (after normalization):

[
  [
    "#type" => "processed_text"
    "#text" => "Hello"
    "#format" => "basic_html"
  ],
  [
    #theme" => "node",
    "#node" => Drupal\node\Entity\Node {},
    "#view_mode" => "teaser"
  ],
  [
    #theme" => "node",
    "#node" => Drupal\node\Entity\Node {},
    "#view_mode" => "teaser"
  ]
]

Or an other rule... If no automatic rule is possible, do nothing.

1. Mikael's: add an option in ComponentSlotsForm

For example, a checkbox to decide if we flat the list or not.