I found this bug when working on MDL-80391, which I think is a similar issue

How to reproduce:

1. Login as admin and go to Site administration > Plugins > Manage editors.
2. Make sure "TinyMCE editor" is enabled and at the top of the list.
3. Now go to Dashboard, enable edit mode and click "Add a block" in the right pane.
4. Select type "Text...".
5. In Content field click Insert > Sample code. A window manager will open with the code sample editor.
6. Try to add the code sample.
7. Verify that you cannot