Hi Frank,
The default toggle button is given a tabindex of 1 so that it will be the first element focused when tabbing through the site, since it appears visually at the top.
This is because the button is actually injected into the document in the footer, then positioned via CSS at the top. If the tabindex were set to 0, it would actually be the last thing you’d focus when tabbing through the page. Generally you want the user to be able to access the menu early on for the best UX.
If you add a custom Toggle within the flow of the document (within your header, for example, rather than using the default toggle), then the tabindex will be set to 0 so that it will be focused within the normal flow.
Hope that makes sense ??