Make tocbot work inside iframes

[felipemarcos]

Problem

The instanceof window.HTMLElement statement returns false when inside an iframe element in very specific situations, this causes the script to stop working thus rendering a blank table of contents.

Solution

I've added instanceof window.parent.HTMLElement to check if an element was created by the parent document. This check only works in iframes in the same domain, so I wrapped it in a try-catch block to prevent errors.

[tscanlin]

Thanks for the fix / PR!!

[tscanlin]

This should be published now. Thanks again! 📦 tocbot@4.21.1