We have many htmleditable_tinymce fields in this SugarCRM and we were using it without any issue but recent upgrade to TinyMCE editor causes this bug. Following are the steps to reproduce this issue:
1. Open record view of any contact.
2. Click pencil icon next to "Description" field.
3. Click "Source code" icon
4. Click inside text area (in Source code popup), it will disappear.
So popup at step 4 should remain there until user closes it using any control(Ok or Cancel button). Please validate this issue in main OnDemand instances.