Missing Scrollbar in h5P IFrame Embedder


We embed a  HTML5-Document with the iFrame Embedder. The document has 2 vertical rows: on the left side table of content and on the right side the text. There is only a scrollbar for the TOC but not for the text. It seems the text is somehow fixed to one page.

When we create the HTML5-Document without TOC, there is no scrollbar at all and page-height is fixed.

Any help is very much appreciated!