Menu is generated again for each page, how does the block cache work?

Question

We have the following layout for a modified navigation menu which takes quite some time to generated. We noticed, that the menu seems to be generated again for each category page, even it is always the same.

           <block name="topmenu_generic"
                   as="topmenu"
                   template="Magento_Theme::html/header/topmenu.phtml"
                   ttl="3600"
            >
                <arguments>
                    <argument name="cache_lifetime" xsi:type="string">3600</argument>
                </arguments>
                <block name="topmenu_mobile"
                       as="topmenu.mobile"
                       template="Magento_Theme::html/header/menu/mobile.phtml"
                ></block>

                <block name="topmenu_desktop"
                       as="topmenu.desktop"
                       template="Magento_Theme::html/header/menu/desktop.phtml"
                ></block>
            </block>

...


        <referenceBlock name="topmenu_generic">
            <arguments>
                <argument name="cache_lifetime" xsi:type="number">-1</argument>
            </arguments>
            <block class="Magento\Theme\Block\Html\Topmenu" name="topmenu.navigation" template="Magento_Theme::html/header/topmenu/navigation.phtml" ttl="3600">
                <block class="Magento\Cms\Block\Block" name="menu_virtual_categories">
                    <arguments>
                        <argument name="block_id" xsi:type="string">menu_virtual_categories</argument>
                    </arguments>
                </block>
                <block class="Magento\Cms\Block\Block" name="menu_cms_pages">
                    <arguments>
                        <argument name="block_id" xsi:type="string">menu_cms_pages</argument>
                    </arguments>
                </block>
            </block>
            <block class="Magento\Store\Block\Switcher" name="languages.mobile" template="Magento_Theme::html/header/topmenu/languages-mobile.phtml" />
        </referenceBlock>

How can we properly cache the menu on the first generation so it is pulled from the cache when a user accesses a page that is not yet in the full page cache?

The cache key info should be this:

public function getCacheKeyInfo()
{
    return [
        'BLOCK_TPL',
        $this->_storeManager->getStore()->getCode(),
        $this->getTemplateFile(),
        'base_url' => $this->getBaseUrl(),
        'template' => $this->getTemplate()
    ];
}

So this does not depend on the URL path, right?

Bonus question for my understanding: If a parent block is cached / has a generic cache key and so on, are all the children also cached or do I have to modify the cache key and lifetime information for each child block?

Answer 1

The culprit is

\Magento\Catalog\Plugin\Block\Topmenu::afterGetCacheKeyInfo

public function afterGetCacheKeyInfo(\Magento\Theme\Block\Html\Topmenu $subject, array $result)
{     
    $activeCategory = $this->getCurrentCategory();
    if ($activeCategory) {
        $result[] = Category::CACHE_TAG . '_' . $activeCategory->getId();
    }

    return $result;
}

This clearly causes the menu to be rendered per-category.