I'm trying the customized MindmapDefaultLayoutProvider (extends
LeftRightProvider) of the book (on page 83).
It looks like the generated diagram editor sticks with the default
provider (Top-to-bottom) if I keep proprity to "Low" (as in the book).
Putting it to "Medium" solved the problem, but, now, after some other
modification to the diagram.custom (e.g., adding the "insert subtopic"
action) "Medium" is not enough... and I had to turned it to "High" to
have the Left-to-Right again...