Indeed, when the first element in a document is a large image, the control gets scrolled to the caret position which is in the bottom-left corner of the image.
I tested the case you mentioned in your update and you are right that the behavior is not as expected. If you set the Document property of the editor in the constructor the DocumentChanged event is actually raised twice. It looks like we use a dispatcher internally in our code and what you can do in order to prevent the document from scrolling is use a construction with two dispatchers as follows:
Please try that and let us know how it goes.