New widget: scrollable card stack
In some cases you need a scrolling list but in the structure of the interface it doesn't really make sense to have a boundary where the items scrolling "out" at the top and bottom disappear. A prominent example is the notification list in Phosh, where the notification cards are "above" the rest of the system area.
For cases like this we need a widget that's basically a scrollview, but instead of cutting off the entire scrolling view at the top and bottom it would only cut off the first and last card internally (similar to a GtkRevealer that follows scrolling), in order to fit onto the available area.
This is an example that does something similar (though in this case the items also scale when they fade back, which isn't necessarily needed):