Book module's BookNavigationBlock passes poorly formatted variable data to the book-all-books-block template

Problem/Motivation

The data passed to the theme_book_all_books_block theme function contain a spurious parent array around the book menu arrays, thus we end up with this markup:

<div id="book-block-menu-0" class="book-block-menu">
  <ul class="menu clearfix">
    <li class="first last collapsed"><a href="/node/1">just some stuff</a></li>
  </ul>
  <ul class="menu clearfix">
    <li class="first last collapsed"><a href="/node/3">sdfserefsf</a></li>
  </ul>
</div>

When we should have this markup:

<div id="book-block-menu-1" class="book-block-menu">
  <ul class="menu clearfix">
    <li class="first last collapsed"><a href="/node/1">just some stuff</a></li>
  </ul>
</div>
<div id="book-block-menu-3" class="book-block-menu">
  <ul class="menu clearfix">
    <li class="first last collapsed"><a href="/node/3">sdfserefsf</a></li>
  </ul>
</div>

Note that the individual menus are wrapped by one div in the wrong example, not individual divs with the correct book menu ids which in this case should be 1 and 3, not 0.

I encountered this issue while doing some accessibility work.

Proposed resolution

This is happening because the variable data passed to the theme function is constructed like this:

if ($book_menus) {
  return array(
    '#theme' => 'book_all_books_block',
    $book_menus,
  );
}

which introduces a secret little [0] indexed array around the $book_menus array.

We can get around this by taking the union of the arrays like so:

if ($book_menus) {
  return array(
    '#theme' => 'book_all_books_block',
  ) + $book_menus;
}

Thus maintaining the individual book menu indices as siblings to the #theme index.

Remaining tasks

Review the patch.

User interface changes

We'll get the right markup.

API changes

None.

Related Issues

#2052473: Add aria-label or aria-describedby attributes to all <nav> elements