The Idea is asking for the rich text ribbon to be disabled during rich text editing. If the ribbon is disabled, how do you plan to Edit the content?
The ideal mode to use the Client is in Full Screen or Maximized View that gives you a better experience, would like to understand why the size is being reduced?
In pre-12, the rich edit toolbars can be used (in the toolbar area of the main window) or you can use the edit menus. From the screenshots, it seems this is still the case, so that would answer Kartik's question. Also, editing the field does not necessarily mean using the formatting options, most times, you want to change the text but not the format.
Allowing to hide the menu/toolbar from inside the field does look like a good idea.
As stated earlier, the Windchill RV&S 13.1 release introduced multiple options, and we have followed-up by bringing an additional None option with the Windchill RV&S 13.2 release. Please review and provide your feedback.