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
                                    global-semanticdb-minor-mode
                                    global-semantic-mru-bookmark-mode
                                    global-cedet-m3-minor-mode))