I have a simple slideshow that rotates images for the prices for the site. Should I have this slideshow continue when a popup is shown? My thoughts, a slideshow could be distracting. However, it could also encourage a user to finish login faster because they see the prizes.
I would say that the slideshow should stop.
The popup becomes the central point of focus you want the user to concentrate on.
/ the user has to do something with the popup to continue the use of the site / continue their journey.
If the slideshow is continuing in the background, it's distracting. Making it harder for the user to complete their goal. It could also frustrate or annoy them, giving them a negative impression. And therefore, more likely to exit.
When you decide about the behaviour of your slideshow you need to think to your users.
It's true that there is only one focus of attention so a slideshow that run under your login make noise but on the other side we have the habituation to the stimulus with which user can ignore the noise and make more attention on One object.
So for example I prefer to have a static slideshow if my users and target are elder or people that don't have a lot experience of the web beacuse this require less energy for login to the user.
On the other site if you have ypinger people as user, the flow of slideshow became an interesting way to promote information. Young people have more energy to manage oveflow information and noise in the right way.
I think the slideshow should stop ideally. I agree with @richardstelmach. It will be distracting when something partially hidden moves and something on the foreground demands my attention.
Apart from that there is another aspect. As I open a pop up, I have a mental image of how the screen behind that pop up is. When I close my pop up and I see an altogether different look, I would be confused.
If I am a veteran user and I am aware of carousels, even then, I would be annoyed to learn that when the pop up was open, I missed a few slides which could have been something of value.
Note that, we are still talking about ideal conditions here. It may not be possible to keep a watch for pop ups and stop slideshow. This needs watchers or event listeners. That coding should still not have any impact on performance of your site.
This case is quite interesting and has also been seen in a lot of places.
Lets take the example of the Google's Android Notification in the Lollipop and above OS. It will show the popup and the rest keeps on working as it is. To take important note here is that the Pop Up will not make the rest of the screen dim, unlike your case where the screen is getting dimmer. In Android since the screen doesn't get dimmer the rest of the work is not affected and you can take control of either the notification or the main work you were doing.
But in you case since the screen is getting dimmer, the pop up gets priority and thus you should stop the slideshow as the user will not be able to see the slides properly.