[Fixed]-TinyMCE not wrapping around entire section

I don’t know TinyMCE at all, but it looks like the editor has a fixed width and height, and the content is overflowing.

TinyMCE may or may not have an API function that abstracts this, but you could try adding the following to the #textarea selector:

overflow: auto;

This would add a scrollbar when any content is clipped, i.e. when it would spill outside the boundary of the container.

I’m not sure if that would go in editor.css specified by the content_css property, or if you need the editor_css property, as per this.