In this examples, there is content that is considered above the fold, such as this text. It is visible on load and appears near the top of the page. It cannot be collapsed, as indicated by the asterisk next to the section title. Additional information is provided in sections located below. These sections are collapsed by default. They do not incur rendering cost since they are display-locked. However, they are still searchable via user interactions. For example, they are expandable by clicking the button next to the section title. They are also expandable via find-in-page.
You can try this out by using find in page to find "github", for example.
Note that render-subtree does not seem to be supported by your browser. This means
that you will not be able to use find-in-page to navigate into collapsed sections.
Here you can find some details which are below the fold. They appear in a collapsed section, meaning that they are not immediately visible on page load.
The collapsed sections have a render-subtree CSS attribute applied with an invisible
token. This means that it induces size containment. Since there is
no competing sizing information, the section collapses "automatically".
Here you can find additional information. Since you are reading this anyway, here you can
find information on render-subtree property:
github page.