How not to "fill up" a view when flagging
I figured out how to manage the problem without the need to write new code. I am posting it here, so that it may help others as well, even if the topic is very specific.

How to manage, that a view doesn't fill up immediately after flagging:


Create a "helper flag" and set the flag link to the helper flag on the nodes of your view.
Create a rule, that the flagging of the helper flag, adds the action of unflagging your original flag.
Put the filter in your view in this way, that not only all nodes are displayed that are unflagged with your original flag, but also those with the helper flag (or-filter)
Create a rules display of this view.
Create a rule that reacts on the event "user has logged out".
Add a view loop from the created rules display and add the action (unflag the nodes". choose the "helper flag" to be unflagged.


That's all. As long as the user stays logged-in, the view is not filled up, when he flags the nodes. As soon as he logs out, the helper flag is unflagged and the view is filled up.