Live Preview and Twig blocks I'm using {% block content %} and a main layout file.

When using live preview the "entire" frame isn't shown (including layout files).

Is there an easy way to get the whole template in?
The problem was my attempt to ajaxify the page.

I had my block content wrapped around
{% if not craft.request.isAjax %}

the page also gets loaded in via Ajax into the control panel, I changed the code to:
{% if not craft.request.isAjax or craft.request.isLivePreview %}

Now it works.