The video/animated preview buttons when in System Settings -> Desktop Behaviour -> Desktop Effects.

Clicking the button gets you a spinning loading indicator, and then sometimes(Looking Glass for example) it's just blank. I guess the video/preview failed to load or no long existed? Whatever the current format is, adding some GIFs here locally to avoid the loading time(5-10 seconds here) should be small on size and give an improved UX. 

The expanded list item doesn't seem to have an intuitive way to collapse back to how it was prior to the preview(clicking the video button again or another list item doesn't collapse). 

Playing with this just now actually froze up System Settings, it became unresponsive, a warning dialog to terminate the window popped up but it's buttons to take action are also unresponsive...which in turned popped up a dialog to kill the unresponsive dialog to kill system settings...also unresponsive, I sent a kill signal to system settings process to resolve. Perhaps the failed video for the effect caused the breakage. 

I can reproduce this with the "Looking Glass" effect. Press the video preview button, wait 10 seconds or so for the spinner to disappear, should be no preview(blank output), click the same video preview button again, it'll go grey and the window will become unresponsive. You can resolve by killing the system settings process, doing this a few times on nvidia seems to have killed compositing too..

The "Looking Glass" effect actually doesn't seem to work for me at all when I've tried it. I also haven't gone through each effect to try the preview button so others may also be affected.

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To summarize
- Previews for some effects are invalid and causing errors (remote content doesn't exist or is unreachable?), unresponsive window happens when using the preview button twice for an effect, using a different effect preview button is fine.
- Local animated GIFs or similar instead of remotely(I'm assuming they're remotely sourced..) sourcing the preview would be nice, 5-10 sec delay, especially for content that doesn't seem to exist is bad UX. Something might be wrong with my install, as no previews seem to be working, but they were fine on my friends fresh install as far as I can recall the other day(eg Wobbly Windows or Zoom).
- The active item in the list does not collapse it's preview window(assuming it's like the info button, clicking the preview button should do this. It will collapse if scrolled far enough out of view.