The new mediaelement video player inside tinymce is awesome, but I found that sometimes it does not load properly.
Here is what the steps I followed :
1. Install a fresh new WordPress 3.9 beta 3 instance on my local machine
2. Create a new post, and click on "Add Media" button
3. Select a video from my hard drive and upload it, then insert it into the post with "Embed Media Player" option selected.
4. The mediaelement player is working well inside tinymce. I can play the video, edit it, add a poster image ...
5. Save the post, then navigate away and come back to the post edition page : the tinymce iframe is void and my browser is constantly loading something (Ubuntu/Chromium). I cannot click anywhere on the page and cannot close the browser tab. On Firefox, I have a message after 30 seconds : "Warning, the script does not respond" : wp-includes/js/jquery.js
I tried with
define('SCRIPT_DEBUG', true); and I do not have any theme or plugin installed.
If I stop the script execution, I can still click on the edit button to change the video settings, and mediaelement player works great in the edition page. It also works great when I display the video on the front-end site, or via the Media menu. But mediaelement player is dead inside tinymce.