One of the features of my plugin is to add a TinyMCE editor instance inside a meta box on the Edit Post screen. This has worked great up until 3.9 and I'm not clear as to what's going wrong.
Here are the relevant files containing the code for the meta box editor (links to files inside Github commit):
What am I missing? This is what I'm given when I load the Edit Post screen:
The contents are there though, which you can see when you highlight inside the textarea:
On WordPress 3.8.1 the code works just fine: