Xinha editor maximalization does not work in IE9

Issue for 7.6 & 7.7:

Reproduce:

• Go to http://cms.demo.onehippo.com/
• Edit a document
• In the description field, enlarge the Xinha field

Observed behavior: the field enlarges, but is empty and unclickable. When you minimize the field again, it remains empty. When closing the document, the text is visible again.

Issue occurs only in Internet Explorer 9.