Fix that allows previewing blocks (named repeaters) in Bolt 3.4+

.bolt.yml

extensions:
   - Bolt\Extension\Foo\Bar\FixPreviewBlocksExtension

composer.json

{
    ...
    "autoload": {
        "psr-4": {
            "Bolt\\Extension\\Foo\\Bar\\": "extensions/bundles/foo/bar/src/"
        }
    }
}

FixPreviewBlocksExtension.php

<?php

namespace Bolt\Extension\Foo\Bar;

use Bolt\Extension\SimpleExtension;

/**
 * @author Xiao-Hu Tai <[email protected]>
 */
class FixPreviewBlocksExtension extends SimpleExtension
{

    /**
     * {@inheritdoc}
     */
    protected function registerTwigFilters()
    {
        return [
            'prefillBlocks' => 'twigPrefillBlocks',
        ];
    }

    /**
     * Helper function to prefill `block` fields with an additional `block` field per item.
     *
     * This is needed for the preview function to work. In templates that makes extensive use
     * of blocks (i.e. testing for which type of 'block' it is), do:
     *
     *     {% set record = record|prefillBlocks %}
     *
     * For a given record, check its contenttype and rewrite all of its block fields, iff
     * the value is an `array` instead of an `object` (e.g. `RepeatingFieldCollection` or
     * `LazyFieldCollection`).
     *
     * This function is probably obsolete once https://github.com/bolt/bolt/pull/7291 is
     * put into stable.
     */
    public function twigPrefillBlocks($record)
    {
        foreach ($record->contenttype['fields'] as $fieldSlug => $fieldSettings) {
            if ($fieldSettings['type'] === 'block'
                && isset($record->values[$fieldSlug])
                && is_array($record->values[$fieldSlug]
            )) {
                $newArray = [];
                foreach ($record->values[$fieldSlug] as $index => $item) {
                    if (! empty($item)) {
                        foreach ($item as $k => &$v) {
                            $v['block'] = $k;
                        }
                        $newArray[] = $v;
                    }
                }
                $record->values[$fieldSlug] = $newArray;
            }
        }

        return $record;
    }

}

See bolt/bolt#7328