Document $attributes, $title_attributes, and $content_attributes template variables

[klonos]

This is follow-up to #2982:

...I saw $content_attributes being printed right after the classes, and I wondered if that variable could also contain additional classes. In that case, shouldn't we be merging those classes, or is it done automagically?

I have tried figuring out what $content_attributes may contain and where it comes from, but it is not documented (hence my search and the d.org issues I came across).

Respective issues for this in d.org https://www.drupal.org/project/drupal/issues/2332087 / https://www.drupal.org/node/569362

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PR by @klonos (rip-off of @drupol's D7 backport patch 😄): ...

[klonos]

...I have started taking a look at things, and have noticed that our block.tpl.php (for starters) has considerably less variables documented, compared to the one in D7. Wondering why is that?

Backdrop https://github.com/backdrop/backdrop/blob/1.x/core/modules/layout/templates/block.tpl.php:

/**
 * @file
 * Template for outputting the default block styling within a Layout.
 *
 * Variables available:
 * - $classes: Array of classes that should be displayed on the block's wrapper.
 * - $title: The title of the block.
 * - $title_prefix/$title_suffix: A prefix and suffix for the title tag. This
 *   is important to print out as administrative links to edit this block are
 *   printed in these variables.
 * - $content: The actual content of the block.
 */

D7 https://git.drupalcode.org/project/drupal/blob/7.x/modules/block/block.tpl.php:

/**
 * @file
 * Default theme implementation to display a block.
 *
 * Available variables:
 * - $block->subject: Block title.
 * - $content: Block content.
 * - $block->module: Module that generated the block.
 * - $block->delta: An ID for the block, unique within each module.
 * - $block->region: The block region embedding the current block.
 * - $classes: String of classes that can be used to style contextually through
 *   CSS. It can be manipulated through the variable $classes_array from
 *   preprocess functions. The default values can be one or more of the
 *   following:
 *   - block: The current template type, i.e., "theming hook".
 *   - block-[module]: The module generating the block. For example, the user
 *     module is responsible for handling the default user navigation block. In
 *     that case the class would be 'block-user'.
 * - $title_prefix (array): An array containing additional output populated by
 *   modules, intended to be displayed in front of the main title tag that
 *   appears in the template.
 * - $title_suffix (array): An array containing additional output populated by
 *   modules, intended to be displayed after the main title tag that appears in
 *   the template.
 *
 * Helper variables:
 * - $classes_array: Array of html class attribute values. It is flattened
 *   into a string within the variable $classes.
 * - $block_zebra: Outputs 'odd' and 'even' dependent on each block region.
 * - $zebra: Same output as $block_zebra but independent of any block region.
 * - $block_id: Counter dependent on each block region.
 * - $id: Same output as $block_id but independent of any block region.
 * - $is_front: Flags true when presented in the front page.
 * - $logged_in: Flags true when the current user is a logged-in member.
 * - $is_admin: Flags true when the current user is an administrator.
 * - $block_html_id: A valid HTML ID and guaranteed unique.
 *
 * @see template_preprocess()
 * @see template_preprocess_block()
 * @see template_process()
 *
 * @ingroup themeable
 */

...was this part of some clean up (I would not want to revert things if this was intentional)?

Pinging @jenlampton

[laryn]

@klonos Those seem to have been removed as part of the commit introducing Layouts:

• backdrop/backdrop@6490d6b