This may actually apply to more popups.
When you click on the "Grade" link from mod/assignment/submissions.php, you get a popup that is a little less than 700px wide. Unfortunately, that popup contains both the breadcrumb and the left column and it uses the incourse layout.
There isn't enough room for the TinyMCE editor to fit into the small space and the breadcrumb is unnecessary.
This grade page should use the popup page layout, or (less appealing) have these elements pulled out.
I seem to recall seeing this on other popup pages as well (perhaps course information link from the front page?) I'll have a look. This might be a wider problem.