Dropdown menus inside TinyMCE dialogs appear behind the dialog

Description

Steps to reproduce:
1. Add the codesample plugin to the editor.
2. Go to edit mode and edit an XHTML property.
3. Click the code sample toolbar icon to open the dialog.
4. Click the drop down to choose a language.

Expected:
The language dropdown menu appears.

Actual:
The language dropdown menu is not visible. If you move the dialog, it is possible to see the language dropdown menu behind the dialog.