I wrote about the undesirable shifting UIs earlier. Here is an alternate perspective on such UIs. There are times when a shifting UI has a purpose. When it does, it takes away the attention of the user to a new menu or a popup for a reason. The user was meant to be looking at the changed UI and it may even be relevant to the user.
News sites often do this type of animation. As the reader reaches the end of an article, they would show a peek into the next article. That is a highly relevant distraction – one that is worthy of the user’s attention.
So, there is a place for these transient UI elements. It is not all bad or all good – the key is, as long as the UI matches the intent of the user or an action that is a natural step for the user, even popups can be good. Random shifts that distract the user from what they want to look at are not cool!