I should have a "live preview" as I edit a Large Text

(User Story, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Editor, UX
Summary: One of my most-desired stories, this would make editing the Default View much easier and more reliable.
This should be a button or something. When pressed, it allows you to select an Instance of this Model (if this is a Model), and splits the pane. On the left is the Large Text editor; on the right is a live-updating preview of what the selected Instance would look like when you apply this View to it. Every time the value is saved, or a certain number of characters have been typed, or simply every few seconds after a change, we call the server to evaluate this QText in the context of the selected Thing, and display that.
There must also be a button to turn the preview off.
The rendered view needs to be height-limited in order to not be inconvenient. It will often have a scrollbar. That's okay.