Thanks for the tips, I will try it out. Since I can't reproduce it at will, I'm not sure when I'll secure the redisplay traces.

>>    The place to start is in cedet/semantic/idle.el.  The outward
>> call to the various work functions purposely does not use
>> 'save-excursion' so tools like completion can move things around
>> for you.
> Or maybe I don't understand what he meant.

I have seen the undesired scrolling with global-semantic-idle-completions-mode disabled. Specifically, with these Semantic submodes:

  (setq semantic-default-submodes '(global-semantic-idle-scheduler-mode