Required text field client side validation fails in TinyMCE

This has been discovered by wuzi1999 while testing moodle.org 3.1 upgrade (MDLSITE-4521).

Reproduce:

1. Choose TinyMCE as you preferred editor
2. Attempt to submit a text in the forum

Expected behaviour: The text is posted when pressing the submit button.

Actual behaviour: The first press of the submit button leads to false validation failure, marking the post text field as required even if the text is present there. Only the second press of the submit button actually submits the form.

Reproduced by me on vanilla 3.1-rc1 installation.